The semiperimeter of a triangle with side lengths a, b, and c is denoted s and is equal to half the perimeter of a triangle: s = (a + b + c)/2.
The semiperimeter is a useful quantity in the calculation of a triangle's area (Heron's formula) and the radius of a triangle's incircle and circumcircle. It is also used to find the contact points, or points of tangency, with the incircle and the three excircles.