Triangle Area Calculator

Use this calculator to find the area and perimeter of a triangle when you know all three sides (SSS). Three lengths form a triangle only if each pair sums to more than the third side (the triangle inequality). If that fails, the sides cannot close and the calculator reports an error.

Perimeter is a + b + c. With s the semi-perimeter (half the perimeter), Heron’s formula gives area = √(s (s − a) (s − b) (s − c)). The height to a chosen base is then 2 × area / that base, if you need it separately.

Use the same length unit for all three sides. Perimeter stays in that unit; area is in that unit squared.

Side a: First side.

Side b: Second side.

Side c: Third side.

Area: Interior of the triangle, from Heron’s formula.

Perimeter: Sum of the three sides.