Do I enter the radius or the diameter of a circle?

Either. The circle, sphere, cylinder and cone each take a radius by default, and a switch beside the field turns it into a diameter. The diameter is halved before it goes into the formula, and the formula line says so: it reads π r², r = d/2. A radius of 5 and a diameter of 10 are the same circle and give the same 78.5398. Switching between radius and diameter converts the currently entered measure so the shape stays the same.

How does the three-sided triangle work without a height?

By Heron's formula. Half the perimeter is s = (a + b + c) / 2, and the area is the square root of s(s − a)(s − b)(s − c). Sides of 3, 4 and 5 give 6. If the two shorter sides do not add up to more than the longest, the sides cannot meet to form a triangle, and the page says so instead of printing zero or a blank.

Which units can I use, and how does the conversion work?

Type the sides in millimetres, centimetres, metres, inches or feet. The area comes out in that unit squared, and the list under it converts the same area to six more units: square millimetres, centimetres, metres, kilometres, inches, feet and yards, acres and hectares between them. The conversions use the exact definitions, so a square foot is exactly 0.09290304 square metres and exactly 144 square inches.

What is the difference between area and surface area?

A flat shape has one area: the space inside its outline. A solid has a surface area: the total area of every face wrapped around it. The five solids here (cube, box, sphere, cylinder, cone) give that total for a closed shape, ends and base included. A cylinder of radius 2 and height 5 has a surface of 2 π r (r + h), which is 87.96, both circular ends counted.

How is the regular polygon worked out?

From the number of sides and the length of one side, with the formula n s² / (4 tan(π/n)). A regular polygon has every side the same length and every corner the same angle, so a hexagon of side 2 is 10.3923 and four sides of 5 give exactly the square's 25. The number of sides has to be a whole number of at least 3.

How many decimal places does the answer have?

The headline shows up to six significant figures and the rows up to twelve, including scientific notation. Browser arithmetic rounds intermediate values. Extremely small or large dimensions that produce zero, overflow or unavailable conversions are refused. A textbook result such as 12 × 8 prints 96.

Can I enter fractions or change units after typing?

Numeric dimensions accept decimals, complete comma groups, scientific notation, fractions such as 3/4 and mixed numbers such as 2 1/2. Choose the measured unit before typing: changing the unit changes the meaning of bare numbers. Written units, coordinates and algebraic expressions are not inputs.

What does the drawing show?

The labelled drawing identifies the dimensions used by the formula; it is schematic rather than to scale. The working and conversion rows contain the numeric result. Saved inputs restore locally without overwriting their original record on load.

Area Calculator

Pick a shape, type its sides in millimetres, centimetres, metres, inches or feet, and read the area with the formula it came from. Ten flat shapes from rectangle to sector, five solids for surface area, and every answer listed in six more units at once, so a room measured in feet still comes out in square metres.

Rectangle · l × w96cm²9,600 mm² · 0.0096 m² · 14.88 in²

Flat

Solid, surface area

Length (cm)

Width (cm)

Unit

Length times width. Any two sides that meet at a corner. The drawing is schematic. Choose the measured unit before typing; changing it sets the meaning of the numbers.

Result summary

Rectangle, length 12 cm, width 8 cm
96 cm²
12 × 8 = 96
9600 mm²
0.0096 m²
14.8800297601 in²
0.10333354 ft²
0.0114815044445 yd²
0.00000237221166208 ac

The workingarea, in cm²
12 × 8 = 96
Square millimetresmm²
9,600
Square metresm²
0.0096
Square inchesin²
14.8800297601
Square feetft²
0.10333354
Square yardsyd²
0.0114815044445
Acresac
0.00000237221166208

The formula is on the stage, and the working is the first row. Every answer here is one textbook formula with your numbers put in, printed so you can check it: a circle is π r², a three-sided triangle is Heron's formula, a regular polygon is n s² / (4 tan(π/n)). The solids give surface area for a closed shape, both ends of a cylinder and the base of a cone included. Conversions use exact definitions, so a square foot is 0.09290304 m² and 43,560 of them make an acre.

Common questions

Do I enter the radius or the diameter of a circle?: Either. The circle, sphere, cylinder and cone each take a radius by default, and a switch beside the field turns it into a diameter. The diameter is halved before it goes into the formula, and the formula line says so: it reads π r², r = d/2. A radius of 5 and a diameter of 10 are the same circle and give the same 78.5398. Switching between radius and diameter converts the currently entered measure so the shape stays the same.
How does the three-sided triangle work without a height?: By Heron's formula. Half the perimeter is s = (a + b + c) / 2, and the area is the square root of s(s − a)(s − b)(s − c). Sides of 3, 4 and 5 give 6. If the two shorter sides do not add up to more than the longest, the sides cannot meet to form a triangle, and the page says so instead of printing zero or a blank.
Which units can I use, and how does the conversion work?: Type the sides in millimetres, centimetres, metres, inches or feet. The area comes out in that unit squared, and the list under it converts the same area to six more units: square millimetres, centimetres, metres, kilometres, inches, feet and yards, acres and hectares between them. The conversions use the exact definitions, so a square foot is exactly 0.09290304 square metres and exactly 144 square inches.
What is the difference between area and surface area?: A flat shape has one area: the space inside its outline. A solid has a surface area: the total area of every face wrapped around it. The five solids here (cube, box, sphere, cylinder, cone) give that total for a closed shape, ends and base included. A cylinder of radius 2 and height 5 has a surface of 2 π r (r + h), which is 87.96, both circular ends counted.
How is the regular polygon worked out?: From the number of sides and the length of one side, with the formula n s² / (4 tan(π/n)). A regular polygon has every side the same length and every corner the same angle, so a hexagon of side 2 is 10.3923 and four sides of 5 give exactly the square's 25. The number of sides has to be a whole number of at least 3.
How many decimal places does the answer have?: The headline shows up to six significant figures and the rows up to twelve, including scientific notation. Browser arithmetic rounds intermediate values. Extremely small or large dimensions that produce zero, overflow or unavailable conversions are refused. A textbook result such as 12 × 8 prints 96.
Can I enter fractions or change units after typing?: Numeric dimensions accept decimals, complete comma groups, scientific notation, fractions such as 3/4 and mixed numbers such as 2 1/2. Choose the measured unit before typing: changing the unit changes the meaning of bare numbers. Written units, coordinates and algebraic expressions are not inputs.
What does the drawing show?: The labelled drawing identifies the dimensions used by the formula; it is schematic rather than to scale. The working and conversion rows contain the numeric result. Saved inputs restore locally without overwriting their original record on load.

Supported formulas use rounded browser numbers and exact unit definitions. Results show up to twelve significant figures; measurements carry their own error.