What is perimeter?
The total length around a 2D shape.
Perimeter Calculator
Find the distance around common plane shapes.
Select a shape
Choose rectangle, triangle, or circle and enter the needed lengths.
About this calculator
How to use Perimeter Calculator, plus the formulas and logic behind the results.
Tool description
Perimeter is the total boundary length. For a circle it is called circumference.
How to use
- Select the shape.
- Enter the required positive lengths.
- Select Calculate.
- Read the perimeter result.
Formulas & reference
Rectangle: P = 2(L + W)Triangle: P = a + b + cCircle: C = 2πr
Perimeter tips
- Add all outer sides
- Rectangle uses opposite sides twice
- Circle uses 2πr
- Units stay consistent with inputs
Calculation logic
Applies the selected shape formula with positivity checks. Circle uses M_PI.
Triangle sides are not validated as a non-degenerate triangle beyond positivity.
Perimeter Calculator FAQ
Common questions about this calculator and how to use the results.
How do you find rectangle perimeter?
P = 2(length + width).
How do you find triangle perimeter?
Add the three side lengths.
Is circle perimeter circumference?
Yes. C = 2πr.
Must sides be positive?
Yes in this calculator.
Does this compute area?
No—use the area calculator for area.