This perimeter calculator finds the distance around a shape — perimeter for polygons, circumference for circles. Pick a shape, enter its sides, and get the boundary length with the formula shown.

Perimeter–

The Perimeter Formulas in One Place

Shape Formula Example (verified)
Rectangle 2(l + w) 2 × (12 + 9) = 42
Square 4 × side 4 × 7 = 28
Triangle a + b + c 3 + 4 + 5 = 12
Circle (circumference) 2πr 2π × 7 = 43.98
Regular polygon n × side hexagon: 6 × 5 = 30

This calculator is part of our Math Calculators collection and the boundary-companion to the Area Calculator.

Perimeter in Real Life

  • Fencing, trim and edging are sold and priced by length — the perimeter is the purchase quantity.
  • The classic confusion: doubling a garden’s sides doubles the fence (perimeter) but quadruples the soil and seed (area). Projects need both numbers; mixing them up is expensive.
  • Running tracks and walking routes are perimeters too — one lap of a standard track is 400 m of boundary around the field.

Circles get the full treatment — diameter, area and the π story — in our Circle Calculator.

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Frequently Asked Questions

How do I find the perimeter of a rectangle?

Add length and width, then double: 2(l + w). A 12 × 9 rectangle has perimeter 42.

What is the perimeter of a circle called?

The circumference: C = 2πr (or πd). Same idea — the distance around — with its own name.

How do I find the perimeter of an irregular shape?

Add every side length. There is no shortcut formula; measure each edge and sum. (Curved boundaries need calculus or a measuring wheel in practice.)

What is the perimeter of a regular hexagon with side 5?

6 × 5 = 30 — for any regular polygon, multiply the side by the number of sides.

Perimeter vs. area — which does fencing need?

Perimeter. Fencing follows the boundary. Area is for what goes inside the boundary — sod, soil, paint on the ground.

Is this perimeter calculator free?

Yes — free, instant, no sign-up.