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Mathematics

Perimeter calculator

Perimeter calculator: enter your values and get an instant free result.

Your result

Complete the fields and calculate.

Calculation method and formula

Rectangle: 2 × (Length / radius / side + Width (rectangle only)); Circle: 2 × π × Length / radius / side; Equilateral triangle: 3 × Length / radius / side

Understand the inputs

Shape
Choose the matching option; it determines the table value or calculation variant.
Length / radius / side
Use a measured value or your own assumption.
Width (rectangle only)
Use a measured value or your own assumption.

Assumptions and limitations

Use the same unit for every length. Area has squared units and volume cubed units. The perimeter triangle is equilateral; circle and sphere formulas use radius, not diameter.

The formula uses the specified units. Displayed numbers are rounded; whole packs, item counts and full time periods may also be rounded within the calculation.

Worked example

Illustrative values. Replace them with your own inputs for your calculation.

Shape
Rectangle
Length / radius / side
5
Width (rectangle only)
4
Example result18 m

The result was calculated directly from your entries.

Model revision: October 17, 2026 · How CleverCalc works

FAQ

Frequently asked questions

How does Perimeter calculator work?+

Rectangle: 2 × (Length / radius / side + Width (rectangle only)); Circle: 2 × π × Length / radius / side; Equilateral triangle: 3 × Length / radius / side. Use the same unit for every length. Area has squared units and volume cubed units. The perimeter triangle is equilateral; circle and sphere formulas use radius, not diameter.

What assumptions should I consider for this result?+

Use the same unit for every length. Area has squared units and volume cubed units. The perimeter triangle is equilateral; circle and sphere formulas use radius, not diameter. The formula uses the specified units. Displayed numbers are rounded; whole packs, item counts and full time periods may also be rounded within the calculation.

How accurate is the result?+

The formula uses the specified units. Displayed numbers are rounded; whole packs, item counts and full time periods may also be rounded within the calculation. Prices, measurements and model assumptions determine how well the result represents your situation.

Is this calculator free?+

Yes. Calculations and PDF export work without an account or registration.

What happens to my inputs?+

Calculations run in your browser. Results are saved locally only when you add them to My results; favourites and history are also local. Shared links contain your values. The currency tool requests the selected currencies from its rate provider without sending your amount.

Toolbox

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