How to use the triangle calculator
Enter the lengths of all three sides. The calculator uses Heron's formula for area and the law of cosines for angles. Results appear instantly.
Triangle formulas
Area (Heron's): s = (a+b+c)/2, Area = √(s·(s−a)·(s−b)·(s−c)). Angles (law of cosines): cos A = (b²+c²−a²)/(2bc). A triangle is valid only if each side is less than the sum of the other two.
FAQ
What inputs are needed?
You need all three side lengths (SSS). The calculator derives everything else: area, perimeter, all three angles, and the triangle type.
What does 'invalid triangle' mean?
A triangle is only possible if the sum of any two sides is greater than the third side (triangle inequality). If your values violate this rule, no triangle exists.