Find the area, perimeter, and type of a triangle from its three sides. Enter the side lengths.
Usage Tip
A 3-4-5 triangle is the classic right triangle, since 3² + 4² = 5².
Type from sides and from the Pythagorean comparison
Comparing the longest side squared to the sum of the other squares tells whether it is right, acute, or obtuse.
Type by sides: equilateral, isosceles, or scalene.
Type by angles: right, acute, or obtuse.
This calculator uses the three side lengths of a triangle to find its area, perimeter, and type. The area comes from Heron’s formula, which uses the semi-perimeter, half the sum of the sides, and needs no height measurement. The type tells you whether the triangle is equilateral, isosceles, or scalene.
Not every set of three lengths makes a triangle. The two shorter sides must add up to more than the longest side, otherwise the sides cannot meet. This is called the triangle inequality. If the squares of the two shorter sides add up to the square of the longest, you have a right triangle.