Pick a shape from the buttons at the top. Enter the measurements asked for, such as radius, side length, or base and height. Press Calculate and you will see the area, perimeter (or circumference), and a live preview drawing of the shape with your values.
Area is the amount of flat space inside a shape. You measure it in square units, like cm² or m². Think of it as how much paint you would need to fill a shape in.
Perimeter is the total distance around the outside of a shape. You measure it in regular units, like cm or m. Think of it as how much fencing you would need to go around a yard. For circles, the equivalent of perimeter is called circumference.
Use area when you need to cover a surface. Use perimeter when you need to go around an edge.
| Shape | Area Formula | Perimeter Formula |
|---|---|---|
| Circle | A = π × r² | C = 2 × π × r |
| Square | A = s² | P = 4 × s |
| Rectangle | A = l × w | P = 2l + 2w |
| Triangle | A = 0.5 × b × h | P = a + b + c |
| Trapezoid | A = 0.5 × (a + b) × h | P = a + b + c + d |
Pi (written as the symbol π) is the ratio of a circle's circumference to its diameter. No matter how big or small the circle is, this ratio is always the same number: approximately 3.14159.
Pi is called an irrational number because its decimal never ends and never repeats. For most calculations you can use 3.14 or the fraction 22/7 as an approximation. This calculator uses the full precision value so your answers are accurate.
