Semiperimeter Calculator

About this calculator

The semiperimeter of a triangle, usually written s, is simply half the sum of its three side lengths: s = (a + b + c) / 2. On its own it is a modest quantity, but it is the key ingredient of Heron's formula for triangle area, Area = √[s(s−a)(s−b)(s−c)], which finds the area from side lengths alone with no need to know any angle.

The semiperimeter also appears in the formula for a triangle's inradius (r = Area / s, the radius of the circle inscribed inside the triangle touching all three sides) and in various other triangle-geometry identities, which is why it is computed as a standalone step before those formulas are applied. This calculator takes the three side lengths and returns the semiperimeter directly.