Perimeter Calculator
Pick a shape, enter its sides, read the perimeter — five shapes including the circle’s circumference, all live in your browser.
How to use
- Choose the shape from the dropdown.
- Fill in the side or radius fields that appear.
- The perimeter updates instantly as you type.
Frequently asked questions
Which shapes can I compute a perimeter for?
Five: square (4 × side), rectangle (2 × (width + height)), circle (circumference = 2πr), triangle (sum of three sides) and any regular polygon (number of sides × side length).
Why is a circle’s perimeter called a circumference?
Historically, the boundary length of a circle got its own name: circumference = 2πr, where π ≈ 3.14159 is the ratio every circle shares between its circumference and diameter. It is still just a perimeter — a curve you can walk along.
Does the triangle check that my sides form a valid shape?
The calculator sums whatever three numbers you enter. For a physically closable triangle, the triangle inequality must hold: each side must be shorter than the sum of the other two — for example 1, 2, 10 cannot close.
What is a regular polygon?
A polygon where all sides are equal (and all angles match, too). Its perimeter is simply the side count times one side — a hexagon with 5 cm sides has a 30 cm perimeter.
Is perimeter the same as area?
No. Perimeter measures the boundary length (a 1-D quantity, in m or ft); area measures the surface enclosed (2-D, in m² or ft²). A 1 m × 100 m strip and a 10 m × 10 m square share no perimeter value even if areas collide — always check which one a task needs.