The total boundary length of a shape, computed for four common shapes on one page.
How it works
Each shape uses its own standard perimeter formula: four times the side for a square, twice the sum of length and width for a rectangle, the sum of all three sides for a triangle, and 2π times the radius (the circumference) for a circle.
What this does not include
This does not include area — for that, use this site’s general area calculator instead, which covers the same and additional shapes from the area side.
How to use this calculator
- Select a shape and enter its dimensions.
A worked example
Square, side 5 → 4 × 5 = 20. Rectangle, length 8 and width 5 → 2 × (8+5) = 26. Triangle, sides 3, 4, 5 → 3+4+5 = 12. Circle, radius 4 → 2π × 4 = 25.1327.
What the formulas are
| Shape | Formula |
|---|---|
| Square | 4 × side |
| Rectangle | 2 × (length + width) |
| Triangle | side a + side b + side c |
| Circle | 2π × radius (its circumference) |
Edge cases worth knowing
The triangle formula needs all three side lengths, not an angle or height — three side lengths that fail the triangle inequality (any two must outweigh the third) don’t describe a real triangle.
A square’s formula is really the rectangle formula with equal sides — 4 × side and 2 × (side + side) give the identical result.
Frequently asked questions
Is a circle’s “perimeter” the same as its circumference?
Yes — circumference is simply the specific term for a circle’s perimeter, its distance around the outside.
Does a square use a special case of the rectangle formula?
Yes — a square is a rectangle where length and width are equal, so its perimeter formula (4 × side) is a simplified version of the rectangle formula.
What’s a practical use for perimeter?
Estimating fencing, trim, or edging material needed to go around a shape’s boundary.
