What is the perimeter of a triangle?

The perimeter is the total length around the triangle — the sum of all three sides: P = a + b + c.

What units does this calculator use?

The calculator is unit-agnostic. Enter values in any consistent unit (meters, feet, inches, etc.) and the result will be in the same unit.

What is the triangle inequality theorem?

For three lengths to form a valid triangle, every pair of sides must sum to more than the third side. If this condition is not met, the calculator will warn you.

Can I find a missing side if I know the perimeter?

Yes. Switch to "Find Missing Side" mode, enter the total perimeter and the two sides you know, and the calculator subtracts them: c = P - a - b.

Does the tool handle decimal values?

Yes. You can enter decimal side lengths (e.g., 3.5, 7.25) and the result is rounded to four decimal places for precision.

Perimeter of a Triangle Calculator

Enter all three sides to find the perimeter, or enter the perimeter and two sides to find the missing third side.

Calculator Inputs

Side a

Side b

Side c

Result

Enter values and press Calculate.

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Summary

This calculator works in two modes: give it all three side lengths and it instantly returns the perimeter (a + b + c), or give it the total perimeter and any two known sides and it solves for the third. It also checks the triangle inequality theorem to flag impossible triangles.

How it works

Choose a calculation mode — "Find Perimeter" or "Find Missing Side".
Enter the required values in the input fields.
The result updates instantly as you type.
Use the Reset button to clear all fields and start over.
Copy the result with one click.

Use cases

Homework and geometry class exercises.
Architecture and construction measurements.
Fencing or border planning for triangular plots.
Game development and 2D geometry calculations.
Verifying hand calculations quickly.
Teaching students about the triangle inequality theorem.

Frequently Asked Questions

Last updated: 2026-05-28 · Reviewed by Nham Vu