Enter three side lengths to calculate a triangle's perimeter.
How to Use the Triangle Perimeter Calculator
Enter the three side lengths a, b, and c. The perimeter is their sum; the semi-perimeter — half that total — is shown alongside it, since it's the number most other triangle formulas (Heron's formula included) actually need.
perimeter = a + b + c
semi-perimeter = perimeter ÷ 2Not every three lengths close into a triangle — each side has to be shorter than the other two combined, or the shape never meets up. This calculator checks that before returning a result, rather than adding up three numbers that don't describe a real triangle.