Perimeter Calculator
Perimeter is the total distance around a shape. Pick your shape, enter the measurements that apply — the other boxes are simply ignored — and get the perimeter and matching area instantly.
Example: with Shape Rectangle · Length / side a / radius 8 · Width / side b (rectangle, triangle) 5 · Side c (triangle only) 6 → Perimeter: 26.00.
Computed by the calculator below using its default values. Change any input to see your own numbers.
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Check it outHow to find the perimeter of any shape
To calculate perimeter, add up every side — that’s the whole idea. The shape formulas are just shortcuts: rectangle P = 2(l + w), so an 8 × 5 room needs 2 × 13 = 26 units of baseboard; square P = 4a; triangle P = a + b + c; and a circle’s perimeter is its circumference, P = 2πr — a radius of 3 gives about 18.85.
Two things trip people up. First, perimeter is a length, measured in ft or m, while area is measured in square units — this tool shows both so you can grab whichever the job needs (fencing uses perimeter; sod uses area). Second, an irregular shape has no shortcut: walk the boundary and sum every segment, and remember two shapes can share an area but have very different perimeters — a 9 × 1 rectangle and a 3 × 3 square both cover 9, but their perimeters are 20 and 12.
How itβs calculated
Rectangle: P = 2 × (length + width), area = length × width. Square: P = 4 × side, area = side². Triangle: P = a + b + c, with the sides checked against the triangle inequality; area comes from Heron’s formula √(s(s−a)(s−b)(s−c)) where s = P ÷ 2. Circle: P = 2πr (the circumference) and area = πr², using π = 3.14159265… Only the fields a given shape needs are read; the rest are ignored.
Results update as you type and are estimates, not professional advice β verify important decisions with a qualified professional.
Common mistakes
- Confusing perimeter with area — perimeter is the distance around (ft, m); area is the surface inside (sq ft, sq m).
- Adding a rectangle’s length and width without doubling — there are two of each side: P = 2(l + w), not l + w.
- Entering triangle sides that can’t close, like 1, 2, 10 — each pair of sides must add to more than the third.
Frequently asked questions
How do you find the perimeter of a shape?
Add the lengths of all its sides. For an 8 × 5 rectangle: 8 + 5 + 8 + 5 = 26 — the formula 2(l + w) is just that sum written compactly.
What is the perimeter of a circle called?
The circumference, C = 2πr. A circle with a 3-unit radius has a circumference of 2 × 3.14159 × 3 ≈ 18.85 units.
Can two shapes have the same area but different perimeters?
Yes. A 9 × 1 rectangle and a 3 × 3 square each cover 9 square units, but their perimeters are 20 and 12. Area never determines perimeter on its own.
How is perimeter used in real projects?
It sizes anything that runs along a boundary: fencing a yard, trim and baseboards, edging a garden bed, or string lights around a patio. Measure in one unit and buy 5–10% extra for cuts.
How do I find the perimeter of an irregular shape?
There’s no shortcut formula — measure each straight segment of the boundary and add them all. Splitting the outline into labeled segments first helps you avoid missing one.