A perimeter calculator finds the distance around a shape. Choose a rectangle, square, triangle or circle, enter the measurements and the calculator returns the perimeter, the area and the steps.
For a circle the perimeter is called the circumference. The perimeter of a rectangle is the most common request, so that is the default shape.
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Perimeter formulas
| Shape | Perimeter | Area |
|---|---|---|
| Rectangle | 2 × (length + width) | length × width |
| Square | 4 × side | side² |
| Triangle | a + b + c | √(s(s − a)(s − b)(s − c)) |
| Circle | 2 × π × r | π × r² |
The perimeter of a rectangle
Add all four sides. Opposite sides are equal, so you can add the length and width and double the result.
P = 2 × (l + w)
The perimeter of a triangle
Add the three sides. To also get the area from three sides, use Heron’s formula with s, half the perimeter.
The circumference of a circle
The circumference is 2 × π × r, where r is the radius, or π × d for a diameter d.
Worked examples
Example 1: a rectangle
A rectangle is 12 long and 7 wide. The perimeter is 2 × (12 + 7) = 38 and the area is 12 × 7 = 84. The diagonal is √(144 + 49) ≈ 13.89.
Example 2: a triangle
A triangle has sides 7, 8 and 9. The perimeter is 24, so s = 12 and the area is √(12 × 5 × 4 × 3) = √720 ≈ 26.83.
Example 3: a circle
A circle has radius 5. The circumference is 2 × π × 5 ≈ 31.42 and the area is π × 25 ≈ 78.54.
Example 4: a square
A square has a side of 9. The perimeter is 36 and the area is 81.
Frequently asked questions
How do you find the perimeter of a rectangle?
Add the length and width, then multiply by 2. For a 12 by 7 rectangle that is 2 × 19 = 38.
What is the difference between perimeter and area?
Perimeter is the distance around the outside of a shape and is measured in units such as cm. Area is the space inside and is measured in square units such as cm².
What is the perimeter of a circle called?
The circumference. It equals 2 × π × the radius.
Can any three lengths make a triangle?
No. Each side must be shorter than the other two added together. The calculator checks this and tells you if the sides cannot form a triangle.
Do I need to use the same units?
Yes. Enter every measurement in the same unit and the results use that unit, with area in square units.