Area and perimeter calculator
Choose a shape, enter its measurements and get the area and perimeter (or circumference) together, with the formula written out.
How the area and perimeter calculator works
Area is the amount of flat space inside a shape, measured in square units. Perimeter is the distance around the edge, measured in ordinary units, and for a circle it has its own name: circumference. People mix the two up constantly, so the results here are labelled with their units.
For a triangle where you only know the three sides, the calculator uses Heron's formula, which saves you having to find the height first.
The formula behind it
- Rectangle: area = l × w; perimeter = 2(l + w)
- Triangle: s = (a+b+c)/2; area = √(s(s-a)(s-b)(s-c))
- Circle: area = πr²; circumference = 2πr
- Parallelogram: area = base × height
A worked example
A circle with radius 5: area = π × 5² = π × 25 = 78.54 square units, and circumference = 2 × π × 5 = 31.42 units.
Questions people ask
Why is area in square units?
Because you are counting how many unit squares fit inside the shape. 5 cm × 4 cm covers 20 square centimetres.
Should I use 3.14 or the pi button?
Use whatever your teacher asks for. The calculator uses the full value of pi, so its answers may differ slightly from 3.14 working.
What if my units are mixed?
Convert everything to the same unit first. Our unit converter can help.
What is the height of a parallelogram?
The straight-up distance between the two parallel sides, not the length of the slanted side.