How to calculate area and perimeter
- Choose the shape (square, rectangle, triangle, circle…) and the length unit.
- Enter the measurements each shape asks for; the fields change depending on the shape.
- See the area highlighted at the top and the perimeter (or circumference) right below.
- Tap “Copy result” to take the values with you.
Area and perimeter formulas
Square with side l: area = l², perimeter = 4l. Rectangle: area = length × width, perimeter = 2 × (length + width). Triangle: area = base × height ÷ 2 (or Heron's formula with the three sides), perimeter = sum of the sides. Circle with radius r: area = π r², circumference = 2 π r.
Trapezoid: area = (longer base + shorter base) ÷ 2 × height. Parallelogram: area = base × height. Rhombus: area = longer diagonal × shorter diagonal ÷ 2, and the side is the square root of (d1/2)² + (d2/2)². Ellipse: area = π a b; its perimeter has no exact formula and Ramanujan's approximation is used.
Examples: a 4 × 3 m rectangle has an area of 12 m² and a perimeter of 14 m. A circle with a radius of 2 cm has an area of 12.566 cm² and a circumference of 12.566 cm. A triangle with sides 3, 4 and 5 has an area of 6 and a perimeter of 12.
Good to know
- The area is shown in square units of the chosen unit (m², cm², ft²…).
- All the measurements of a shape must be in the same unit.
- The perimeter of the ellipse is a very precise approximation, not an exact value.
Frequently asked questions
What is the difference between area and perimeter?
The area is the surface the shape covers (in square units); the perimeter is the length of its outline.
How do I calculate the area of a rectangle?
Multiply the length by the width. A 4 × 3 rectangle has an area of 12 square units.
How do I find the area of a triangle if I know the three sides?
Enter them in the base, side b and side c fields and leave the height empty: the tool uses Heron's formula.
Which unit do I use for the area?
The same square unit as the measurements: if you measure in meters, the area is in square meters (m²).
Are my measurements saved?
No. The calculation is done in your browser and nothing is sent to any server.