Triangle Calculator (Right Triangle & SSS Solver, with Diagram)
Use this free triangle calculator to solve a triangle two ways. In right-triangle mode, enter the two legs to get the hypotenuse, both acute angles, the area and the perimeter. In SSS mode, enter all three sides to get every angle (Law of Cosines), the area (Heron’s formula) and the perimeter — with a diagram drawn to scale.
How to use the triangle calculator
- Pick a mode: Right triangle (two legs) or SSS (three sides).
- Enter your values.
- Read the solved sides, angles, area and perimeter, and see the triangle drawn to scale.
The methods
For a right triangle we use the Pythagorean theorem (c = √(a² + b²)) and inverse tangent for the angles. For a general triangle we use the Law of Cosines to find each angle and Heron’s formula for the area. A set of three sides only forms a triangle if each side is shorter than the sum of the other two.
Frequently asked questions
How do you find the angles of a triangle from three sides?
Use the Law of Cosines: cos(C) = (a² + b² − c²) / (2ab). The calculator solves all three angles this way and shows the steps.
What is Heron’s formula?
With s = (a + b + c)/2, the area is √(s(s−a)(s−b)(s−c)). It gives the area of any triangle from its three sides.
Why does it say my sides don’t form a triangle?
Because of the triangle inequality: each side must be less than the sum of the other two. If one side is too long, no triangle exists.
Read the full lesson: learn the method step by step.
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