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.