Enter the three side lengths of a triangle to calculate perimeter, semiperimeter and area. The calculator also checks whether the three sides can form a valid triangle.
Right triangles, areas, slopes, distances and common coordinate-geometry formulas.
The perimeter is the total distance around the triangle. Add the three side lengths together.
For side lengths 3, 4 and 5, the perimeter is 3 + 4 + 5 = 12. The semiperimeter is 6.
Three positive lengths form a triangle only when every pair of sides adds to more than the remaining side. The calculator checks this automatically before returning a result.
The semiperimeter is half the perimeter and is useful in Heron’s formula for finding area when all three side lengths are known.
Add all three sides: P = a + b + c.
No. The sum of any two sides must be greater than the third side.
Perimeter uses the same linear unit as the side lengths, such as inches, feet, centimeters or meters.
See how formulas, sources and limitations are handled across ERICH Calculators.