Perimeter Calculator
Pick a shape below to instantly get its perimeter.
Perimeter Formulas by Shape
- Rectangle: 2(l + w)
- Square: 4s
- Triangle: a + b + c
- Circle (circumference): 2πr
- Regular polygon: number of sides × side length
Working with a regular hexagon or a trapezoid specifically? See the dedicated hexagon or trapezoid calculators.
A common mistake with irregular shapes is skipping a side because it's hidden in a diagram or assumed to match another — every side has to be counted once, even short connecting edges. For regular polygons, remember the multiplier is the number of sides, not the number of angles (though for a simple polygon those two counts are always equal).
Worked examples
A rectangle with length 9 and width 5.
Perimeter = 2(9 + 5) = 2 × 14 = 28
A regular pentagon with side length 7.
Perimeter = 5 × 7 = 35
Keep going
- A square's perimeter is the simplest case this calculator covers; the square calculator adds area and diagonal length for that same shape. Square Calculator
- A circle's perimeter is its circumference, a formula this calculator already includes; the circle calculator adds area and diameter around that same measurement. Circle Calculator
Frequently Asked Questions
What is perimeter?
Perimeter is the total distance around the outside edge of a two-dimensional shape — add up the length of every side.
What's the difference between perimeter and circumference?
They're the same idea — the distance around a shape's outer edge — but "circumference" is the specific term used only for circles.
How do you find the perimeter of an irregular shape?
Add up the length of every individual side, even if they're all different lengths. The formulas here are shortcuts for shapes where the sides follow a predictable pattern (all equal, or opposite sides equal).
Does a bigger perimeter always mean a bigger area?
No. Shapes with the exact same perimeter can enclose very different amounts of area — for a fixed perimeter, a circle always encloses the most area of any shape, while a long, thin rectangle can enclose almost none. Perimeter and area measure different things: one is the length of the boundary, the other is the space inside it.
Related Calculators
- Sphere Calculator — Geometry & Trigonometry
- Cylinder Calculator — Geometry & Trigonometry
- Cone Calculator — Geometry & Trigonometry
- Triangle Area Calculator — Geometry & Trigonometry