The semiperimeter of a geometric figure is one half of the perimeter, or $\frac{P}{2}$, where $P$ is the total perimeter of a figure. It is typically denoted $s$. In a triangle, it has uses in formulas for the lengths relating the excenter and incenter.

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The semiperimeter has many uses in geometric formulas. Perhaps the simplest is $A=rs$, where $A$ is the area of a triangle and $r$ is the triangle's inradius (that is, the radius of the circle inscribed in the triangle).

Two other well-known examples of formulas involving the semiperimeter are Heron's formula and Brahmagupta's formula.