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- Area is the amount of flat space inside a shape, measured in square units such as cm², m² or ft².
- This calculator covers rectangles, squares, triangles, circles, trapezoids, parallelograms and ellipses, and gives the perimeter wherever it can.
- Results are converted to square meters, square feet, square yards, acres and hectares at the same time.
- Use one unit for every measurement. Mixing feet and inches in the same shape gives a wrong answer.
How to use the area calculator
Choose a shape
Pick a rectangle, square, triangle, circle, trapezoid, parallelogram or ellipse. The boxes change to match.
Choose a unit
Select millimeters, centimeters, meters, inches, feet or yards. Every measurement uses this unit.
Enter the measurements
Type each length. For triangles, trapezoids and parallelograms, use the straight, perpendicular height.
Read the results
You get the area, the perimeter or circumference, and the area in m², ft², yd², acres and hectares.
Area formulas
Area formulas for each shape
Rectangle: A = length × width
Square: A = side²
Triangle: A = 1/2 × base × height
Triangle (three sides): A = √(s(s − a)(s − b)(s − c))
Circle: A = π × r²
Trapezoid: A = 1/2 × (b1 + b2) × height
Parallelogram: A = base × height
Ellipse: A = π × a × b
In the three-side formula, s is half of the perimeter: s = (a + b + c) ÷ 2.
Area is measured in square units because it counts the squares that fit inside the shape. A rectangle 10 cm by 5 cm holds 50 squares that are 1 cm on each side, so its area is 50 cm². Doubling every side of a shape makes its area four times as large, not twice as large.
Worked examples
Example 1: a room in square feet
A room is 12 ft by 15 ft.
Flooring is usually sold by the square foot, so 180 ft² is the number to take to the shop. Many people add 5 to 10 percent extra for waste.
Example 2: a round pond
A pond has a radius of 4 ft.
The radius is half of the diameter, so a pond 8 ft across has a radius of 4 ft.
Example 3: a triangle from three sides
A garden bed has sides of 7 m, 8 m and 9 m.
Three sides only make a triangle if any two of them add up to more than the third.
Example 4: a building plot
A plot measures 50 m by 30 m.
Land is often quoted in acres or hectares, so the conversions save a second calculation.
Use the right measurement
The most common mistake is using the slanted side where the formula needs the height. In a triangle, parallelogram or trapezoid, the height is measured at a right angle to the base. The slanted side is always longer, so using it would make the area too large. Slanted sides only matter for the perimeter, which is why they are optional boxes here.
Area unit conversions
Common area conversions
Exact or rounded to six significant figures.
A square meter is a square with sides of exactly one meter.
Where an area calculator helps
Flooring and paint
Find how much floor, carpet, tile or wall you need to cover.
Land and property
Convert a plot size between square meters, acres and hectares.
Gardening
Plan lawns, beds and ponds, and work out how much soil or seed to buy.
School and design
Check geometry homework and size posters, tables and layouts.
Once you know the area, multiplying by a depth gives the volume. The volume calculator does this for boxes, cylinders, cones and more. You can also use the percentage calculator to add extra for waste, or the square root calculator to find the side of a square from its area.
Area calculator FAQ
What is the formula for the area of a rectangle?
Multiply the length by the width. A rectangle 10 cm long and 5 cm wide has an area of 50 cm².
How do I find the area of a triangle with three sides?
Use the three-side formula. Find s, half of the perimeter, then take the square root of s × (s − a) × (s − b) × (s − c). Sides of 3, 4 and 5 give an area of 6.
What is the difference between area and perimeter?
Area is the space inside a shape, in square units. Perimeter is the distance around its edge, in ordinary units such as feet or meters.
How many square feet are in a square meter?
One square meter is about 10.7639 square feet. Going the other way, one square foot is about 0.092903 square meters.
How many square meters are in an acre?
One acre is 4,046.8564 square meters, which is 43,560 square feet. One hectare is 10,000 square meters, or about 2.471 acres.
Why does the calculator ask for height instead of the side?
For triangles, trapezoids and parallelograms, area uses the straight, perpendicular height. The slanted side is longer, so using it would overstate the area. Slanted sides are only needed for the perimeter.