Enter the three sides of a triangle to get its area, perimeter and type.
The method behind this calculator
Give this calculator the three side lengths and it returns the area using Heron’s formula, the perimeter, the height, and whether the triangle is right, equilateral, isosceles or scalene.
It also checks the sides actually form a valid triangle.
A worked example
On the values this calculator opens with, the area is 6. Underneath, Perimeter comes out at 12 and Type at Right. Change any field and every figure updates as you type.
Using the calculator
- Enter the three side lengths.
- Read the area and perimeter.
- Check the triangle type and height.
Formula reference
s = (a + b + c) ÷ 2
area = √(s(s−a)(s−b)(s−c)) (Heron’s formula)
FAQ
How do I find a triangle’s area from three sides?
Use Heron’s formula: compute the semi-perimeter, then the square root of s(s−a)(s−b)(s−c).
When is it a valid triangle?
Each side must be shorter than the sum of the other two. Otherwise the sides can’t close into a triangle.
How is the type decided?
By comparing sides and checking the Pythagorean relationship — equal sides, or a²+b²=c² for a right triangle.