Perimeter Calculator
Calculate the perimeter of common shapes: rectangle, triangle, and circle. Switch between shapes with tabs.
Perimeter
—
Perimeter Formulas
Rectangle: P = 2 × (length + width)
Triangle: P = a + b + c
Circle (Circumference): P = 2 × π × r
Triangle: P = a + b + c
Circle (Circumference): P = 2 × π × r
Rectangle: length=8, width=5 → P = 2×(8+5) = 26 units
Triangle: sides 3, 4, 5 → P = 3+4+5 = 12 units
Circle: radius=7 → P = 2π×7 ≈ 43.98 units
Triangle: sides 3, 4, 5 → P = 3+4+5 = 12 units
Circle: radius=7 → P = 2π×7 ≈ 43.98 units
Perimeter Summary
| Shape | Formula | Notes |
|---|---|---|
| Rectangle | 2(l + w) | Opposite sides are equal |
| Square | 4s | Special rectangle with equal sides |
| Triangle | a + b + c | Sum of all three sides |
| Circle | 2πr | Also called circumference |
Frequently Asked Questions
What is perimeter?
Perimeter is the total distance around the outside boundary of a 2D shape. Think of it as the length of fence needed to enclose a yard.
What units is perimeter measured in?
Perimeter is measured in linear units (cm, m, inches, feet) — not square units. It measures length, not area.
How is perimeter different from area?
Perimeter measures the distance around a shape; area measures the space inside it. Two shapes can have the same perimeter but very different areas.
Related Calculators
Last updated: 2026-08-08