Triangle Perimeter Calculator

About this calculator

A triangle’s perimeter is simply the sum of its three side lengths, P = a + b + c — but not every three lengths actually form a triangle. The triangle inequality theorem requires that the sum of any two sides be greater than the third; if it isn’t, the “triangle” can’t physically close.

This calculator adds your three sides to get the perimeter and checks that inequality automatically, flagging invalid combinations before you build on a wrong answer. It’s used in basic geometry coursework, and just as often in practical settings like construction and fencing layout, where knowing the total edge length of a triangular plot or truss is a quick first step in material estimating.