This article is about the geometric quantity. For other uses see Area (disambiguation). The combined area of these three shapes is between 15 and 16 squares.

Area is a quantity that expresses the extent of a two-dimensional surface or shape in the plane. 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 (m2) which is the area of a square whose sides are one metre long.1 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.2 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.3

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 a 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.4 In analysis the area of a subset of the plane is defined using Lebesgue measure5 though not every subset is measurable. In general area in higher mathematics is seen as a special case of volume for two-dimensional regions. Contents 1 Formal definition 2 Units 2.1 Conversions 2.2 Other units 3 Basic area formulae 3.1 Rectangles 3.2 Dissection formulae 3.3 Circles 3.4 Surface area 4 List of formulae 5 Additional formulae 5.1 Areas of 2-dimensional figures 5.2 Area in calculus 5.3 Surface area of 3-dimensional figures 5.3.1 General formula 6 Minimization 7 See also 8 References 8.1 Notes 9 External links Formal definition See also: Jordan measure

An approach to defining what is meant by area is through axioms. For example we may define area as a function a 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 . If S and T are in M then so are and and also . If S and T are in M with 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. . If there is a unique number c such that for all such step regions S and T then a(Q) c.

It can be proved that such a area function actually exists. (See for example Elementary Geometry from an Advanced Standpoint by Edwin Moise.) 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 measure 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. The conversion between two square units is the square of the conversion between the corresponding length units. For example since 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 kilometer 1000000 square meters 1 square meter 10000 square centimetres 1000000 square millimetres 1 square centimetre 100 square millimetres 1 square yard 9 square feet 1 square mile 3097600 square yards 27878400 square feet In addition 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 See also: Category:Units of area 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 10000 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 4840 square yards 43560 square feet. An acre is approximately 40% of a hectare. Basic area formulae 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 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 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 formulae Most other simple formulae 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 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:  (triangle). Similar arguments can be used to find area formulae for the trapezoid and the rhombus as well as more complicated polygons. 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 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 and approximate parallelogram. The height of this parallelogram is r and the width is half the circumference of the circle or r. Thus the total area of the circle is r r or r2: A r2  (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 r2 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: Surface area Archimedes showed that the surface area and volume of a sphere is exactly 2/3 of the area and volume of the surrounding cylindrical surface. Most basic formulae 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: because the surface of 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 4r2  (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. List of formulae Common formulae for area: Shape Formula Variables Regular triangle (equilateral triangle) s is the length of one side of the triangle. Triangle s is half the perimeter a b and c are the length of each side. Triangle a and b are any two sides and C is the angle between them. Triangle b and h are the base and altitude (measured perpendicular to the base) respectively. Square s is the length of one side of the square. Rectangle l and w are the lengths of the rectangle's sides (length and width). Rhombus a and b are the lengths of the two diagonals of the rhombus. Parallelogram b is the length of the base and h is the perpendicular height. Trapezoid a and b are the parallel sides and h the distance (height) between the parallels. Regular hexagon s is the length of one side of the hexagon. Regular octagon s is the length of one side of the octagon. Regular polygon s is the side length and n is the number of sides. a is the apothem or the radius of an inscribed circle in the polygon and p is the perimeter of the polygon. Circle r is the radius and d the diameter. Circular sector r and are the radius and angle (in radians) respectively. Ellipse a and b are the semi-major and semi-minor axes respectively. Total surface area of a Cylinder r and h are the radius and height respectively. Lateral surface area of a cylinder r and h are the radius and height respectively. Total surface area of a Cone r and l are the radius and slant height respectively. Lateral surface area of a cone r and l are the radius and slant height respectively. Total surface area of a Sphere r and d are the radius and diameter respectively. Total surface area of an ellipsoid   See the article. Total surface area of a Pyramid B is the base area P is the base perimeter and L is the slant height. Square to circular area conversion A is the area of the square in square units. Circular to square area conversion C is the area of the circle in circular units. The above calculations show how to find the area of many common shapes. The area of irregular polygons can be calculated using the "Surveyor's formula".6 Additional formulae Areas of 2-dimensional figures a triangle: (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: (where a b c are the sides of the triangle and is half of its perimeter) If an angle and its two included sides are given then areaabsinC 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 (x1y2+ x2y3+ x3y1 - x2y1- x3y2- x1y3) all divided by 2. 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 (x1y1) (x2y2) (x3y 3). 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 Infinitesimal 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: 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 The area between two graphs can be evaluated by calculating the difference between the integrals of the two functions 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). an area bounded by a function r r() expressed in polar coordinates is . the area enclosed by a parametric curve with endpoints is given by the line integrals (see Green's theorem) or the z-component of Surface area of 3-dimensional figures cube: 6s2 where s is the length of the top side rectangular box: the length divided by height cone: where r is the radius of the circular base and h is the height. That can also be rewritten as r2 + rl where r is the radius and l is the slant height of the cone. r2 is the base area while rl is the lateral surface area of the cone. prism: 2 Area of Base + Perimeter of Base Height General formula The general formula for the surface area of the graph of a continuously differentiable function z f(xy) where and D is a region in the xy-plane with the smooth boundary: Even more general formula for the area of the graph of a parametric surface in the vector form where is a continuously differentiable vector function of : 4 Minimization 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. See also Equi-areal mapping Integral Orders of magnitude (area)A list of areas by size. Perimeter Volume References Notes Bureau International des Poids et Mesures Mark de Berg Marc van Kreveld Mark Overmars and Otfried Schwarzkopf (2000) Computational Geometry (2nd revised ed.) Springer-Verlag ISBN 3-540-65620-0  Chapter 3: Polygon Triangulation: pp.4561. Boyer Carl B. (1959). A History of the Calculus and Its Conceptual Development. Dover. ISBN 486606094.  a b do Carmo Manfredo. Differential Geometry of Curves and Surfaces. Prentice-Hall 1976. Page 98. Walter Rudin Real and Complex Analysis McGraw-Hill 1966 ISBN 0-07-100276-6. http://www.maa.org/pubs/Calcarticles/ma063.pdf External links Wikimedia Commons has media related to: Area Look up area in Wiktionary the free dictionary. Weisstein Eric W. "Area" from MathWorld. Area formulas Conversion cable diameter to circle cross-sectional area and vice versa