Volume

From Wikipedia, the free encyclopedia

Volume
A measuring cup can be used to measure volumes of liquids. This cup measures volume in units of cups, fluid ounces, and millilitres.
Common symbols	V
SI unit	Cubic metre [m3]
Other units	Litre, Fluid ounce, gallon, quart, pint, tsp, fluid dram, in3, yd3, barrel
In SI base units	1 m3
Dimension	L3

Volume is the quantity of three-dimensional space enclosed by a closed surface, for example, the space that a substance (solid, liquid, gas, or plasma) or shape occupies or contains. Volume is often quantified numerically using the SI derived unit, the cubic metre. The volume of a container is generally understood to be the capacity of the container; i. e., the amount of fluid (gas or liquid) that the container could hold, rather than the amount of space the container itself displaces. Three dimensional mathematical shapes are also assigned volumes. Volumes of some simple shapes, such as regular, straight-edged, and circular shapes can be easily calculated using arithmetic formulas. Volumes of complicated shapes can be calculated with integral calculus if a formula exists for the shape's boundary. One-dimensional figures (such as lines) and two-dimensional shapes (such as squares) are assigned zero volume in the three-dimensional space.

The volume of a solid (whether regularly or irregularly shaped) can be determined by fluid displacement. Displacement of liquid can also be used to determine the volume of a gas. The combined volume of two substances is usually greater than the volume of just one of the substances. However, sometimes one substance dissolves in the other and in such cases the combined volume is not additive.

In differential geometry, volume is expressed by means of the volume form, and is an important global Riemannian invariant. In thermodynamics, volume is a fundamental parameter, and is a conjugate variable to pressure.

Units

Volume measurements from the 1914 The New Student's Reference Work.

Approximate conversion to metric (mL)

	Imp.	U.S.
		Liquid	Dry
Gill	142	118	138
Pint	568	473	551
Quart	1137	946	1101
Gallon	4546	3785	4405

Any unit of length gives a corresponding unit of volume: the volume of a cube whose sides have the given length. For example, a cubic centimetre (cm³) is the volume of a cube whose sides are one centimetre (1 cm) in length.

In the International System of Units (SI), the standard unit of volume is the cubic metre (m³). The metric system also includes the litre (L) as a unit of volume, where one litre is the volume of a 10-centimetre cube. Thus

1 litre = (10 cm)³ = 1000 cubic centimetres = 0.001 cubic metres,

so

1 cubic metre = 1000 litres.

Small amounts of liquid are often measured in millilitres, where

1 millilitre = 0.001 litres = 1 cubic centimetre.

In the same way, large amounts can be measured in megalitres, where

1 million litres = 1000 cubic metres = 1 megalitre.

Various other traditional units of volume are also in use, including the cubic inch, the cubic foot, the cubic yard, the cubic mile, the teaspoon, the tablespoon, the fluid ounce, the fluid dram, the gill, the pint, the quart, the gallon, the minim, the barrel, the cord, the peck, the bushel, the hogshead, the acre-foot and the board foot.

Related terms

Capacity is defined by the Oxford English Dictionary as "the measure applied to the content of a vessel, and to liquids, grain, or the like, which take the shape of that which holds them". (The word capacity has other unrelated meanings, as in e.g. capacity management.) Capacity is not identical in meaning to volume, though closely related; the capacity of a container is always the volume in its interior. Units of capacity are the SI litre and its derived units, and Imperial units such as gill, pint, gallon, and others. Units of volume are the cubes of units of length. In SI the units of volume and capacity are closely related: one litre is exactly 1 cubic decimetre, the capacity of a cube with a 10 cm side. In other systems the conversion is not trivial; the capacity of a vehicle's fuel tank is rarely stated in cubic feet, for example, but in gallons (an imperial gallon fills a volume of 0.1605 cu ft).

The density of an object is defined as the ratio of the mass to the volume. The inverse of density is specific volume which is defined as volume divided by mass. Specific volume is a concept important in thermodynamics where the volume of a working fluid is often an important parameter of a system being studied.

The volumetric flow rate in fluid dynamics is the volume of fluid which passes through a given surface per unit time (for example cubic meters per second [m³ s⁻¹]).

Volume in calculus

In calculus, a branch of mathematics, the volume of a region D in R³ is given by a triple integral of the constant function ${\displaystyle f(x,y,z)=1}$ and is usually written as:

${\displaystyle \iiint \limits _{D}1\,dx\,dy\,dz.}$

The volume integral in cylindrical coordinates is

${\displaystyle \iiint \limits _{D}r\,dr\,d\theta \,dz,}$

and the volume integral in spherical coordinates (using the convention for angles with ${\displaystyle \theta }$ as the azimuth and ${\displaystyle \phi }$ measured from the polar axis; see more on conventions) has the form

${\displaystyle \iiint \limits _{D}\rho ^{2}\sin \phi \,d\rho \,d\theta \,d\phi .}$

Volume formulas

Shape	Volume formula	Variables
Cube	${\displaystyle a^{3}\;}$	a = length of any side (or edge)
Circular Cylinder	${\displaystyle \pi r^{2}h\;}$	r = radius of circular base, h = height
Prism	${\displaystyle Bh}$	B = area of the base, h = height
Cuboid	${\displaystyle lwh}$	l = length, w = width, h = height
Triangular prism	${\displaystyle {\frac {1}{2}}bhl}$	b = base length of triangle, h = height of triangle, l = length of prism or distance between the triangular bases
Triangular prism (with given lengths of three sides)	${\displaystyle {\frac {1}{4}}h{\sqrt {-a^{4}+2(ab)^{2}+2(ac)^{2}-b^{4}+2(bc)^{2}-c^{4}}}}$	a, b, and c = lengths of sides h = height of the triangular prism
Sphere	${\displaystyle {\frac {4}{3}}\pi r^{3}={\frac {1}{6}}\pi d^{3}}$	r = radius of sphere d = diameter of sphere which is the integral of the surface area of a sphere
Ellipsoid	${\displaystyle {\frac {4}{3}}\pi abc}$	a, b, c = semi-axes of ellipsoid
Torus	${\displaystyle \left(\pi r^{2}\right)\left(2\pi R\right)=2\pi ^{2}Rr^{2}}$	r = minor radius (radius of the tube), R = major radius (distance from center of tube to center of torus)
Pyramid	${\displaystyle {\frac {1}{3}}Bh}$	B = area of the base, h = height of pyramid
Square pyramid	${\displaystyle {\frac {1}{3}}s^{2}h\;}$	s = side length of base, h = height
Rectangular pyramid	${\displaystyle {\frac {1}{3}}lwh}$	l = length, w = width, h = height
Cone	${\displaystyle {\frac {1}{3}}\pi r^{2}h}$	r = radius of circle at base, h = distance from base to tip or height
Regular tetrahedron	${\displaystyle {{\sqrt {2}} \over 12}a^{3}\,}$	Edge length, a
Parallelepiped	${\displaystyle abc{\sqrt {K}}}$ ${\displaystyle {\begin{aligned}K=1&+2\cos(\alpha )\cos(\beta )\cos(\gamma )\\&-\cos ^{2}(\alpha )-\cos ^{2}(\beta )-\cos ^{2}(\gamma )\end{aligned}}}$	a, b, and c are the parallelepiped edge lengths, and α, β, and γ are the internal angles between the edges
Any volumetric sweep (calculus required)	${\displaystyle \int _{a}^{b}A(h)\,\mathrm {d} h}$	h = any dimension of the figure, A(h) = area of the cross-sections perpendicular to h described as a function of the position along h. a and b are the limits of integration for the volumetric sweep. (This will work for any figure if its cross-sectional area can be determined from h).
Any rotated figure (washer method; calculus required)	${\displaystyle \pi \int _{a}^{b}\left({\left[R_{O}(x)\right]}^{2}-{\left[R_{I}(x)\right]}^{2}\right)\mathrm {d} x}$	${\displaystyle R_{O}}$ and ${\displaystyle R_{I}}$ are functions expressing the outer and inner radii of the function, respectively.

Volume ratios for a cone, sphere and cylinder of the same radius and height

A cone, sphere and cylinder of radius r and height h

The above formulas can be used to show that the volumes of a cone, sphere and cylinder of the same radius and height are in the ratio 1 : 2 : 3, as follows.

Let the radius be r and the height be h (which is 2r for the sphere), then the volume of cone is

${\displaystyle {\frac {1}{3}}\pi r^{2}h={\frac {1}{3}}\pi r^{2}\left(2r\right)=\left({\frac {2}{3}}\pi r^{3}\right)\times 1,}$

the volume of the sphere is

${\displaystyle {\frac {4}{3}}\pi r^{3}=\left({\frac {2}{3}}\pi r^{3}\right)\times 2,}$

while the volume of the cylinder is

${\displaystyle \pi r^{2}h=\pi r^{2}(2r)=\left({\frac {2}{3}}\pi r^{3}\right)\times 3.}$

The discovery of the 2 : 3 ratio of the volumes of the sphere and cylinder is credited to Archimedes.

Volume formula derivations

Sphere

The volume of a sphere is the integral of an infinite number of infinitesimally small circular disks of thickness dx. The calculation for the volume of a sphere with center 0 and radius r is as follows.
The surface area of the circular disk is ${\displaystyle \pi r^{2}}$.

The radius of the circular disks, defined such that the x-axis cuts perpendicularly through them, is

${\displaystyle y={\sqrt {r^{2}-x^{2}}}}$

or

${\displaystyle z={\sqrt {r^{2}-x^{2}}}}$

where y or z can be taken to represent the radius of a disk at a particular x value.
Using y as the disk radius, the volume of the sphere can be calculated as

${\displaystyle \int _{-r}^{r}\pi y^{2}\,dx=\int _{-r}^{r}\pi \left(r^{2}-x^{2}\right)\,dx.}$

Now

${\displaystyle \int _{-r}^{r}\pi r^{2}\,dx-\int _{-r}^{r}\pi x^{2}\,dx=\pi \left(r^{3}+r^{3}\right)-{\frac {\pi }{3}}\left(r^{3}+r^{3}\right)=2\pi r^{3}-{\frac {2\pi r^{3}}{3}}.}$

Combining yields ${\displaystyle V={\frac {4}{3}}\pi r^{3}.}$

This formula can be derived more quickly using the formula for the sphere's surface area, which is ${\displaystyle 4\pi r^{2}}$. The volume of the sphere consists of layers of infinitesimally thin spherical shells, and the sphere volume is equal to

${\displaystyle \int _{0}^{r}4\pi r^{2}\,dr={\frac {4}{3}}\pi r^{3}.}$

Cone

The cone is a type of pyramidal shape. The fundamental equation for pyramids, one-third times base times altitude, applies to cones as well.

However, using calculus, the volume of a cone is the integral of an infinite number of infinitesimally thin circular disks of thickness dx. The calculation for the volume of a cone of height h, whose base is centered at (0, 0, 0) with radius r, is as follows.

The radius of each circular disk is r if x = 0 and 0 if x = h, and varying linearly in between—that is,

${\displaystyle r{\frac {h-x}{h}}.}$

The surface area of the circular disk is then

${\displaystyle \pi \left(r{\frac {h-x}{h}}\right)^{2}=\pi r^{2}{\frac {(h-x)^{2}}{h^{2}}}.}$

The volume of the cone can then be calculated as

${\displaystyle \int _{0}^{h}\pi r^{2}{\frac {(h-x)^{2}}{h^{2}}}dx,}$

and after extraction of the constants

${\displaystyle {\frac {\pi r^{2}}{h^{2}}}\int _{0}^{h}(h-x)^{2}dx}$

Integrating gives us

${\displaystyle {\frac {\pi r^{2}}{h^{2}}}\left({\frac {h^{3}}{3}}\right)={\frac {1}{3}}\pi r^{2}h.}$

Polyhedron

Volume in differential geometry

In differential geometry, a branch of mathematics, a volume form on a differentiable manifold is a differential form of top degree (i.e., whose degree is equal to the dimension of the manifold) that is nowhere equal to zero. A manifold has a volume form if and only if it is orientable. An orientable manifold has infinitely many volume forms, since multiplying a volume form by a non-vanishing function yields another volume form. On non-orientable manifolds, one may instead define the weaker notion of a density. Integrating the volume form gives the volume of the manifold according to that form.

An oriented pseudo-Riemannian manifold has a natural volume form. In local coordinates, it can be expressed as

${\displaystyle \omega ={\sqrt {|g|}}\,dx^{1}\wedge \dots \wedge dx^{n},}$

where the ${\displaystyle dx^{i}}$ are 1-forms that form a positively oriented basis for the cotangent bundle of the manifold, and ${\displaystyle g}$ is the determinant of the matrix representation of the metric tensor on the manifold in terms of the same basis.

Volume in thermodynamics

In thermodynamics, the volume of a system is an important extensive parameter for describing its thermodynamic state. The specific volume, an intensive property, is the system's volume per unit of mass. Volume is a function of state and is interdependent with other thermodynamic properties such as pressure and temperature. For example, volume is related to the pressure and temperature of an ideal gas by the ideal gas law.

Area

From Wikipedia, the free encyclopedia

Area
Common symbols	A
SI unit	Square metre [m2]
In SI base units	1 m2

The combined area of these three shapes is approximately 15.57 squares.

Area is the quantity that expresses the extent of a two-dimensional figure or shape, or planar lamina, in the plane. Surface area is its analog on the two-dimensional surface of a three-dimensional object. Area can be understood as the amount of material with a given thickness that would be necessary to fashion a model of the shape, or the amount of paint necessary to cover the surface with a single coat. It is the two-dimensional analog of the length of a curve (a one-dimensional concept) or the volume of a solid (a three-dimensional concept).

The area of a shape can be measured by comparing the shape to squares of a fixed size. In the International System of Units (SI), the standard unit of area is the square metre (written as m²), which is the area of a square whose sides are one metre long. A shape with an area of three square metres would have the same area as three such squares. In mathematics, the unit square is defined to have area one, and the area of any other shape or surface is a dimensionless real number.

There are several well-known formulas for the areas of simple shapes such as triangles, rectangles, and circles. Using these formulas, the area of any polygon can be found by dividing the polygon into triangles. For shapes with curved boundary, calculus is usually required to compute the area. Indeed, the problem of determining the area of plane figures was a major motivation for the historical development of calculus.

For a solid shape such as a sphere, cone, or cylinder, the area of its boundary surface is called the surface area. Formulas for the surface areas of simple shapes were computed by the ancient Greeks, but computing the surface area of a more complicated shape usually requires multivariable calculus.

Area plays an important role in modern mathematics. In addition to its obvious importance in geometry and calculus, area is related to the definition of determinants in linear algebra, and is a basic property of surfaces in differential geometry. In analysis, the area of a subset of the plane is defined using Lebesgue measure, though not every subset is measurable. In general, area in higher mathematics is seen as a special case of volume for two-dimensional regions.

Area can be defined through the use of axioms, defining it as a function of a collection of certain plane figures to the set of real numbers. It can be proved that such a function exists.

Formal definition

An approach to defining what is meant by "area" is through axioms. "Area" can be defined as a function from a collection M of special kind of plane figures (termed measurable sets) to the set of real numbers which satisfies the following properties:

• For all S in M, a(S) ≥ 0.
• If S and T are in M then so are S ∪ T and S ∩ T, and also a(S∪T) = a(S) + a(T) − a(S∩T).
• If S and T are in M with S ⊆ T then T − S is in M and a(T−S) = a(T) − a(S).
• If a set S is in M and S is congruent to T then T is also in M and a(S) = a(T).
• Every rectangle R is in M. If the rectangle has length h and breadth k then a(R) = hk.
• Let Q be a set enclosed between two step regions S and T. A step region is formed from a finite union of adjacent rectangles resting on a common base, i.e. S ⊆ Q ⊆ T. If there is a unique number c such that a(S) ≤ c ≤ a(T) for all such step regions S and T, then a(Q) = c.

It can be proved that such an area function actually exists.

Units

A square metre quadrat made of PVC pipe.

Every unit of length has a corresponding unit of area, namely the area of a square with the given side length. Thus areas can be measured in square metres (m²), square centimetres (cm²), square millimetres (mm²), square kilometres (km²), square feet (ft²), square yards (yd²), square miles (mi²), and so forth. Algebraically, these units can be thought of as the squares of the corresponding length units.

The SI unit of area is the square metre, which is considered an SI derived unit.

Conversions

Although there are 10 mm in 1 cm, there are 100 mm² in 1 cm².

Calculation of the area of a square whose length and width are 1 metre would be:

1 metre x 1 metre = 1 m²

and so, a rectangle with different sides (say length of 3 metres and width of 2 metres) would have an area in square units that can be calculated as:

3 metres x 2 metres = 6 m². This is equivalent to 6 million square millimetres. Other useful conversions are:

• 1 square kilometre = 1,000,000 square metres
• 1 square metre = 10,000 square centimetres = 1,000,000 square millimetres
• 1 square centimetre = 100 square millimetres.

Non-metric units

In non-metric units, the conversion between two square units is the square of the conversion between the corresponding length units.

1 foot = 12 inches,

the relationship between square feet and square inches is

1 square foot = 144 square inches,

where 144 = 12² = 12 × 12. Similarly:

• 1 square yard = 9 square feet
• 1 square mile = 3,097,600 square yards = 27,878,400 square feet

In addition, conversion factors include:

• 1 square inch = 6.4516 square centimetres
• 1 square foot = 0.09290304 square metres
• 1 square yard = 0.83612736 square metres
• 1 square mile = 2.589988110336 square kilometres

Other units including historical

There are several other common units for area. The are was the original unit of area in the metric system, with:

• 1 are = 100 square metres

Though the are has fallen out of use, the hectare is still commonly used to measure land:

• 1 hectare = 100 ares = 10,000 square metres = 0.01 square kilometres

Other uncommon metric units of area include the tetrad, the hectad, and the myriad.

The acre is also commonly used to measure land areas, where

• 1 acre = 4,840 square yards = 43,560 square feet.

An acre is approximately 40% of a hectare.

On the atomic scale, area is measured in units of barns, such that:

• 1 barn = 10⁻²⁸ square meters.

The barn is commonly used in describing the cross-sectional area of interaction in nuclear physics.
In India,

• 20 dhurki = 1 dhur
• 20 dhur = 1 khatha
• 20 khata = 1 bigha
• 32 khata = 1 acre

History

Circle area

In the 5th century BCE, Hippocrates of Chios was the first to show that the area of a disk (the region enclosed by a circle) is proportional to the square of its diameter, as part of his quadrature of the lune of Hippocrates, but did not identify the constant of proportionality. Eudoxus of Cnidus, also in the 5th century BCE, also found that the area of a disk is proportional to its radius squared.

Subsequently, Book I of Euclid's Elements dealt with equality of areas between two-dimensional figures. The mathematician Archimedes used the tools of Euclidean geometry to show that the area inside a circle is equal to that of a right triangle whose base has the length of the circle's circumference and whose height equals the circle's radius, in his book Measurement of a Circle. (The circumference is 2πr, and the area of a triangle is half the base times the height, yielding the area πr² for the disk.) Archimedes approximated the value of π (and hence the area of a unit-radius circle) with his doubling method, in which he inscribed a regular triangle in a circle and noted its area, then doubled the number of sides to give a regular hexagon, then repeatedly doubled the number of sides as the polygon's area got closer and closer to that of the circle (and did the same with circumscribed polygons).

Swiss scientist Johann Heinrich Lambert in 1761 proved that π, the ratio of a circle's area to its squared radius, is irrational, meaning it is not equal to the quotient of any two whole numbers. In 1794 French mathematician Adrien-Marie Legendre proved that π² is irrational; this also proves that π is irrational. In 1882, German mathematician Ferdinand von Lindemann proved that π is transcendental (not the solution of any polynomial equation with rational coefficients), confirming a conjecture made by both Legendre and Euler.

Triangle area

Heron (or Hero) of Alexandria found what is known as Heron's formula for the area of a triangle in terms of its sides, and a proof can be found in his book, Metrica, written around 60 CE. It has been suggested that Archimedes knew the formula over two centuries earlier, and since Metrica is a collection of the mathematical knowledge available in the ancient world, it is possible that the formula predates the reference given in that work.

In 499 Aryabhata, a great mathematician-astronomer from the classical age of Indian mathematics and Indian astronomy, expressed the area of a triangle as one-half the base times the height in the Aryabhatiya (section 2.6).

A formula equivalent to Heron's was discovered by the Chinese independently of the Greeks. It was published in 1247 in Shushu Jiuzhang ("Mathematical Treatise in Nine Sections"), written by Qin Jiushao.

Quadrilateral area

In the 7th century CE, Brahmagupta developed a formula, now known as Brahmagupta's formula, for the area of a cyclic quadrilateral (a quadrilateral inscribed in a circle) in terms of its sides. In 1842 the German mathematicians Carl Anton Bretschneider and Karl Georg Christian von Staudt independently found a formula, known as Bretschneider's formula, for the area of any quadrilateral.

General polygon area

The development of Cartesian coordinates by René Descartes in the 17th century allowed the development of the surveyor's formula for the area of any polygon with known vertex locations by Gauss in the 19th century.

Areas determined using calculus

The development of integral calculus in the late 17th century provided tools that could subsequently be used for computing more complicated areas, such as the area of an ellipse and the surface areas of various curved three-dimensional objects.

Area formulas

Polygon formulas

For a non-self-intersecting (simple) polygon, the Cartesian coordinates ${\displaystyle (x_{i},y_{i})}$ (i=0, 1, ..., n-1) of whose n vertices are known, the area is given by the surveyor's formula:

${\displaystyle A={\frac {1}{2}}|\sum _{i=0}^{n-1}(x_{i}y_{i+1}-x_{i+1}y_{i})|}$

where when i=n-1, then i+1 is expressed as modulus n and so refers to 0.

Rectangles

The area of this rectangle is lw.

The most basic area formula is the formula for the area of a rectangle. Given a rectangle with length l and width w, the formula for the area is:

A = lw (rectangle).

That is, the area of the rectangle is the length multiplied by the width. As a special case, as l = w in the case of a square, the area of a square with side length s is given by the formula:

A = s² (square).

The formula for the area of a rectangle follows directly from the basic properties of area, and is sometimes taken as a definition or axiom. On the other hand, if geometry is developed before arithmetic, this formula can be used to define multiplication of real numbers.

Equal area figures.

Dissection, parallelograms, and triangles

Main articles: Triangle § Computing the area of a triangle, and Parallelogram § Area formula

Most other simple formulas for area follow from the method of dissection. This involves cutting a shape into pieces, whose areas must sum to the area of the original shape.

For an example, any parallelogram can be subdivided into a trapezoid and a right triangle, as shown in figure to the left. If the triangle is moved to the other side of the trapezoid, then the resulting figure is a rectangle. It follows that the area of the parallelogram is the same as the area of the rectangle:

A = bh  (parallelogram).

Two equal triangles.

However, the same parallelogram can also be cut along a diagonal into two congruent triangles, as shown in the figure to the right. It follows that the area of each triangle is half the area of the parallelogram:

${\displaystyle A={\frac {1}{2}}bh}$  (triangle).

Similar arguments can be used to find area formulas for the trapezoid as well as more complicated polygons.

Area of curved shapes

Circles

A circle can be divided into sectors which rearrange to form an approximate parallelogram.

Main article: Area of a circle

The formula for the area of a circle (more properly called the area enclosed by a circle or the area of a disk) is based on a similar method. Given a circle of radius r, it is possible to partition the circle into sectors, as shown in the figure to the right. Each sector is approximately triangular in shape, and the sectors can be rearranged to form an approximate parallelogram. The height of this parallelogram is r, and the width is half the circumference of the circle, or πr. Thus, the total area of the circle is πr²:

A = πr²  (circle).

Though the dissection used in this formula is only approximate, the error becomes smaller and smaller as the circle is partitioned into more and more sectors. The limit of the areas of the approximate parallelograms is exactly πr², which is the area of the circle.

This argument is actually a simple application of the ideas of calculus. In ancient times, the method of exhaustion was used in a similar way to find the area of the circle, and this method is now recognized as a precursor to integral calculus. Using modern methods, the area of a circle can be computed using a definite integral:

${\displaystyle A\;=\;2\int _{-r}^{r}{\sqrt {r^{2}-x^{2}}}\,dx\;=\;\pi r^{2}.}$

Ellipses

The formula for the area enclosed by an ellipse is related to the formula of a circle; for an ellipse with semi-major and semi-minor axes x and y the formula is:

${\displaystyle A=\pi xy.}$

Surface area

Archimedes showed that the surface area of a sphere is exactly four times the area of a flat disk of the same radius, and the volume enclosed by the sphere is exactly 2/3 of the volume of a cylinder of the same height and radius.

Most basic formulas for surface area can be obtained by cutting surfaces and flattening them out. For example, if the side surface of a cylinder (or any prism) is cut lengthwise, the surface can be flattened out into a rectangle. Similarly, if a cut is made along the side of a cone, the side surface can be flattened out into a sector of a circle, and the resulting area computed.

The formula for the surface area of a sphere is more difficult to derive: because a sphere has nonzero Gaussian curvature, it cannot be flattened out. The formula for the surface area of a sphere was first obtained by Archimedes in his work On the Sphere and Cylinder. The formula is:

A = 4πr²  (surface area of sphere),

where r is the radius of the sphere. As with the formula for the area of a circle, any derivation of this formula inherently uses methods similar to calculus.

General formulas

Areas of 2-dimensional figures

• A triangle: ${\displaystyle {\tfrac {1}{2}}Bh}$ (where B is any side, and h is the distance from the line on which B lies to the other vertex of the triangle). This formula can be used if the height h is known. If the lengths of the three sides are known then Heron's formula can be used: ${\displaystyle {\sqrt {s(s-a)(s-b)(s-c)}}}$ where a, b, c are the sides of the triangle, and ${\displaystyle s={\tfrac {1}{2}}(a+b+c)}$ is half of its perimeter. If an angle and its two included sides are given, the area is ${\displaystyle {\tfrac {1}{2}}ab\sin(C)}$ where C is the given angle and a and b are its included sides. If the triangle is graphed on a coordinate plane, a matrix can be used and is simplified to the absolute value of ${\displaystyle {\tfrac {1}{2}}(x_{1}y_{2}+x_{2}y_{3}+x_{3}y_{1}-x_{2}y_{1}-x_{3}y_{2}-x_{1}y_{3})}$. This formula is also known as the shoelace formula and is an easy way to solve for the area of a coordinate triangle by substituting the 3 points (x₁,y₁), (x₂,y₂), and (x₃,y₃). The shoelace formula can also be used to find the areas of other polygons when their vertices are known. Another approach for a coordinate triangle is to use calculus to find the area.
• A simple polygon constructed on a grid of equal-distanced points (i.e., points with integer coordinates) such that all the polygon's vertices are grid points: ${\displaystyle i+{\frac {b}{2}}-1}$, where i is the number of grid points inside the polygon and b is the number of boundary points. This result is known as Pick's theorem.

Area in calculus

Integration can be thought of as measuring the area under a curve, defined by f(x), between two points (here a and b).

The area between two graphs can be evaluated by calculating the difference between the integrals of the two functions

• The area between a positive-valued curve and the horizontal axis, measured between two values a and b (b is defined as the larger of the two values) on the horizontal axis, is given by the integral from a to b of the function that represents the curve:

${\displaystyle A=\int _{a}^{b}f(x)\,dx.}$

• The area between the graphs of two functions is equal to the integral of one function, f(x), minus the integral of the other function, g(x):

${\displaystyle A=\int _{a}^{b}(f(x)-g(x))\,dx,}$ where ${\displaystyle f(x)}$ is the curve with the greater y-value.

• An area bounded by a function r = r(θ) expressed in polar coordinates is:

${\displaystyle A={1 \over 2}\int r^{2}\,d\theta .}$

• The area enclosed by a parametric curve ${\displaystyle {\vec {u}}(t)=(x(t),y(t))}$ with endpoints ${\displaystyle {\vec {u}}(t_{0})={\vec {u}}(t_{1})}$ is given by the line integrals:

${\displaystyle \oint _{t_{0}}^{t_{1}}x{\dot {y}}\,dt=-\oint _{t_{0}}^{t_{1}}y{\dot {x}}\,dt={1 \over 2}\oint _{t_{0}}^{t_{1}}(x{\dot {y}}-y{\dot {x}})\,dt}$

(see Green's theorem) or the z-component of

${\displaystyle {1 \over 2}\oint _{t_{0}}^{t_{1}}{\vec {u}}\times {\dot {\vec {u}}}\,dt.}$

Bounded area between two quadratic functions

To find the bounded area between two quadratic functions, we subtract one from the other to write the difference as

${\displaystyle f(x)-g(x)=ax^{2}+bx+c=a(x-\alpha )(x-\beta )}$

where f(x) is the quadratic upper bound and g(x) is the quadratic lower bound. Define the discriminant of f(x)-g(x) as

${\displaystyle \Delta =b^{2}-4ac.}$

By simplifying the integral formula between the graphs of two functions (as given in the section above) and using Vieta's formula, we can obtain

${\displaystyle A={\frac {\Delta {\sqrt {\Delta }}}{6a^{2}}}={\frac {a}{6}}(\beta -\alpha )^{3},\qquad a\neq 0.}$

The above remains valid if one of the bounding functions is linear instead of quadratic.

Surface area of 3-dimensional figures

• Cone: ${\displaystyle \pi r\left(r+{\sqrt {r^{2}+h^{2}}}\right)}$, where r is the radius of the circular base, and h is the height. That can also be rewritten as ${\displaystyle \pi r^{2}+\pi rl}$ or ${\displaystyle \pi r(r+l)\,\!}$ where r is the radius and l is the slant height of the cone. ${\displaystyle \pi r^{2}}$ is the base area while ${\displaystyle \pi rl}$ is the lateral surface area of the cone.
• cube: ${\displaystyle 6s^{2}}$, where s is the length of an edge.
• cylinder: ${\displaystyle 2\pi r(r+h)}$, where r is the radius of a base and h is the height. The 2${\displaystyle \pi }$r can also be rewritten as ${\displaystyle \pi }$ d, where d is the diameter.
• prism: 2B + Ph, where B is the area of a base, P is the perimeter of a base, and h is the height of the prism.
• pyramid: ${\displaystyle B+{\frac {PL}{2}}}$, where B is the area of the base, P is the perimeter of the base, and L is the length of the slant.
• rectangular prism: ${\displaystyle 2(\ell w+\ell h+wh)}$, where ${\displaystyle \ell }$ is the length, w is the width, and h is the height.

General formula for surface area

The general formula for the surface area of the graph of a continuously differentiable function ${\displaystyle z=f(x,y),}$ where ${\displaystyle (x,y)\in D\subset \mathbb {R} ^{2}}$ and ${\displaystyle D}$ is a region in the xy-plane with the smooth boundary:

${\displaystyle A=\iint _{D}{\sqrt {\left({\frac {\partial f}{\partial x}}\right)^{2}+\left({\frac {\partial f}{\partial y}}\right)^{2}+1}}\,dx\,dy.}$

An even more general formula for the area of the graph of a parametric surface in the vector form ${\displaystyle \mathbf {r} =\mathbf {r} (u,v),}$ where ${\displaystyle \mathbf {r} }$ is a continuously differentiable vector function of ${\displaystyle (u,v)\in D\subset \mathbb {R} ^{2}}$ is:

${\displaystyle A=\iint _{D}\left|{\frac {\partial \mathbf {r} }{\partial u}}\times {\frac {\partial \mathbf {r} }{\partial v}}\right|\,du\,dv.}$

List of formulas

Additional common formulas for area:

Shape	Formula	Variables
Regular triangle (equilateral triangle)	${\displaystyle {\frac {\sqrt {3}}{4}}s^{2}\,\!}$	${\displaystyle s}$ is the length of one side of the triangle.
Triangle	${\displaystyle {\sqrt {s(s-a)(s-b)(s-c)}}\,\!}$	${\displaystyle s}$ is half the perimeter, ${\displaystyle a}$, ${\displaystyle b}$ and ${\displaystyle c}$ are the length of each side.
Triangle	${\displaystyle {\tfrac {1}{2}}ab\sin(C)\,\!}$	${\displaystyle a}$ and ${\displaystyle b}$ are any two sides, and ${\displaystyle C}$ is the angle between them.
Triangle	${\displaystyle {\tfrac {1}{2}}bh\,\!}$	${\displaystyle b}$ and ${\displaystyle h}$ are the base and altitude (measured perpendicular to the base), respectively.
Isosceles triangle	${\displaystyle {\frac {1}{2}}b{\sqrt {a^{2}-{\frac {b^{2}}{4}}}}={\frac {b}{4}}{\sqrt {4a^{2}-b^{2}}}}$	${\displaystyle a}$ is the length of one of the two equal sides and ${\displaystyle b}$ is the length of a different side.
Rhombus/Kite	${\displaystyle {\tfrac {1}{2}}ab}$	${\displaystyle a}$ and ${\displaystyle b}$ are the lengths of the two diagonals of the rhombus or kite.
Parallelogram	${\displaystyle bh\,\!}$	${\displaystyle b}$ is the length of the base and ${\displaystyle h}$ is the perpendicular height.
Trapezoid	${\displaystyle {\frac {(a+b)h}{2}}\,\!}$	${\displaystyle a}$ and ${\displaystyle b}$ are the parallel sides and ${\displaystyle h}$ the distance (height) between the parallels.
Regular hexagon	${\displaystyle {\frac {3}{2}}{\sqrt {3}}s^{2}\,\!}$	${\displaystyle s}$ is the length of one side of the hexagon.
Regular octagon	${\displaystyle 2(1+{\sqrt {2}})s^{2}\,\!}$	${\displaystyle s}$ is the length of one side of the octagon.
Regular polygon	${\displaystyle {\frac {1}{4}}nl^{2}\cdot \cot(\pi /n)\,\!}$	${\displaystyle l}$ is the side length and ${\displaystyle n}$ is the number of sides.
Regular polygon	${\displaystyle {\frac {1}{4n}}p^{2}\cdot \cot(\pi /n)\,\!}$	${\displaystyle p}$ is the perimeter and ${\displaystyle n}$ is the number of sides.
Regular polygon	${\displaystyle {\frac {1}{2}}nR^{2}\cdot \sin(2\pi /n)=nr^{2}\tan(\pi /n)\,\!}$	${\displaystyle R}$ is the radius of a circumscribed circle, ${\displaystyle r}$ is the radius of an inscribed circle, and ${\displaystyle n}$ is the number of sides.
Regular polygon	${\displaystyle {\tfrac {1}{2}}ap={\tfrac {1}{2}}nsa\,\!}$	${\displaystyle n}$ is the number of sides, ${\displaystyle s}$ is the side length, ${\displaystyle a}$ is the apothem, or the radius of an inscribed circle in the polygon, and ${\displaystyle p}$ is the perimeter of the polygon.
Circle	${\displaystyle \pi r^{2}\ {\text{or}}\ {\frac {\pi d^{2}}{4}}\,\!}$	${\displaystyle r}$ is the radius and ${\displaystyle d}$ the diameter.
Circular sector	${\displaystyle {\frac {\theta }{2}}r^{2}\ {\text{or}}\ {\frac {L\cdot r}{2}}\,\!}$	${\displaystyle r}$ and ${\displaystyle \theta }$ are the radius and angle (in radians), respectively and ${\displaystyle L}$ is the length of the perimeter.
Ellipse	${\displaystyle \pi ab\,\!}$	${\displaystyle a}$ and ${\displaystyle b}$ are the semi-major and semi-minor axes, respectively.
Total surface area of a cylinder	${\displaystyle 2\pi r(r+h)\,\!}$	${\displaystyle r}$ and ${\displaystyle h}$ are the radius and height, respectively.
Lateral surface area of a cylinder	${\displaystyle 2\pi rh\,\!}$	${\displaystyle r}$ and ${\displaystyle h}$ are the radius and height, respectively.
Total surface area of a sphere	${\displaystyle 4\pi r^{2}\ {\text{or}}\ \pi d^{2}\,\!}$	${\displaystyle r}$ and ${\displaystyle d}$ are the radius and diameter, respectively.
Total surface area of a pyramid	${\displaystyle B+{\frac {PL}{2}}\,\!}$	${\displaystyle B}$ is the base area, ${\displaystyle P}$ is the base perimeter and ${\displaystyle L}$ is the slant height.
Total surface area of a pyramid frustum	${\displaystyle B+{\frac {PL}{2}}\,\!}$	${\displaystyle B}$ is the base area, ${\displaystyle P}$ is the base perimeter and ${\displaystyle L}$ is the slant height.
Square to circular area conversion	${\displaystyle {\frac {4}{\pi }}A\,\!}$	${\displaystyle A}$ is the area of the square in square units.
Circular to square area conversion	${\displaystyle {\frac {\pi }{4}}C\,\!}$	${\displaystyle C}$ is the area of the circle in circular units.

The above calculations show how to find the areas of many common shapes.

The areas of irregular polygons can be calculated using the "Surveyor's formula".

Relation of area to perimeter

The isoperimetric inequality states that, for a closed curve of length L (so the region it encloses has perimeter L) and for area A of the region that it encloses,

${\displaystyle 4\pi A\leq L^{2},}$

and equality holds if and only if the curve is a circle. Thus a circle has the largest area of any closed figure with a given perimeter.

At the other extreme, a figure with given perimeter L could have an arbitrarily small area, as illustrated by a rhombus that is "tipped over" arbitrarily far so that two of its angles are arbitrarily close to 0° and the other two are arbitrarily close to 180°.

For a circle, the ratio of the area to the circumference (the term for the perimeter of a circle) equals half the radius r. This can be seen from the area formula πr² and the circumference formula 2πr.
The area of a regular polygon is half its perimeter times the apothem (where the apothem is the distance from the center to the nearest point on any side).

Fractals

Doubling the edge lengths of a polygon multiplies its area by four, which is two (the ratio of the new to the old side length) raised to the power of two (the dimension of the space the polygon resides in). But if the one-dimensional lengths of a fractal drawn in two dimensions are all doubled, the spatial content of the fractal scales by a power of two that is not necessarily an integer. This power is called the fractal dimension of the fractal.

Area bisectors

There are an infinitude of lines that bisect the area of a triangle. Three of them are the medians of the triangle (which connect the sides' midpoints with the opposite vertices), and these are concurrent at the triangle's centroid; indeed, they are the only area bisectors that go through the centroid. Any line through a triangle that splits both the triangle's area and its perimeter in half goes through the triangle's incenter (the center of its incircle). There are either one, two, or three of these for any given triangle.

Any line through the midpoint of a parallelogram bisects the area.

All area bisectors of a circle or other ellipse go through the center, and any chords through the center bisect the area. In the case of a circle they are the diameters of the circle.

Optimization

Given a wire contour, the surface of least area spanning ("filling") it is a minimal surface. Familiar examples include soap bubbles.

The question of the filling area of the Riemannian circle remains open.

The circle has the largest area of any two-dimensional object having the same perimeter.

A cyclic polygon (one inscribed in a circle) has the largest area of any polygon with a given number of sides of the same lengths.

A version of the isoperimetric inequality for triangles states that the triangle of greatest area among all those with a given perimeter is equilateral.

The triangle of largest area of all those inscribed in a given circle is equilateral; and the triangle of smallest area of all those circumscribed around a given circle is equilateral.

The ratio of the area of the incircle to the area of an equilateral triangle, ${\displaystyle {\frac {\pi }{3{\sqrt {3}}}}}$, is larger than that of any non-equilateral triangle.

The ratio of the area to the square of the perimeter of an equilateral triangle, ${\displaystyle {\frac {1}{12{\sqrt {3}}}},}$ is larger than that for any other triangle.