Perimeter Calculator
Choose a shape, enter its side lengths and get the perimeter, the distance all the way around. Show the work lists each addition or multiplication.
Last updated November 4, 2026
Above 0.
Above 0.
Result
Perimeter = 26
| Measure | Value |
|---|---|
| Shape | Rectangle |
| Perimeter | 26 |
Show the work
Formula: P = 2 × (length + width)
- Length 8, width 5.
- P = 2 × (8 + 5) = 26.
Result: Perimeter = 26
Perimeter formulas
rectangle 2(l + w)
square 4s
triangle a + b + c
circle 2πr
regular polygon n × s
trapezoid a + b + c + d
Perimeter is the sum of all side lengths. For shapes with equal sides it shortens to a multiplication: a square is 4 times its side and a regular polygon is the number of sides times one side.
A circle has no corners, so its perimeter is called the circumference: 2 × π × radius, or π × diameter.
The calculator checks that the shape can exist. A triangle needs each side shorter than the other two added together. A trapezoid needs legs long enough to span the difference between its two parallel bases.
See how we calculate for the formulas, sources and review schedule behind every calculator.
How to use the Perimeter Calculator
- Pick the shape.
- Enter the side lengths. For a circle, choose radius or diameter first.
- Choose a unit if you want it shown in the result.
- Read the perimeter, then open Show the work to see each step.
Worked example
A rectangle 8 long and 5 wide has perimeter 2 × (8 + 5) = 26. More examples:
| Shape | Sides entered | Perimeter |
|---|---|---|
| Rectangle | 8 and 5 | 26 |
| Square | 6 | 24 |
| Triangle | 3, 4, 5 | 12 |
| Circle | radius 5 | 31.4159 |
| Regular hexagon | 6 sides of 4 | 24 |
| Trapezoid | 10, 6, 5, 5 | 26 |
Frequently asked questions
What is the difference between perimeter and area?
Perimeter measures the distance around a shape in length units. Area measures the surface inside it in square units.
How do I find the perimeter of a circle?
Use the circumference formula: 2 × π × radius, or π × diameter. A circle of radius 5 has a circumference of about 31.4159.
Why does the calculator say my triangle is impossible?
Each side of a triangle must be shorter than the other two added together. Sides 1, 2 and 3 fail because 1 + 2 equals 3, which leaves no room to close the shape.
Does the unit change the answer?
No. The unit is only a label. Enter all sides in the same unit, and the perimeter comes out in that unit.