How to calculate triangle
s = (a+b+c)÷2 · Area = √(s(s−a)(s−b)(s−c)) · cos A = (b²+c²−a²)÷2bc
Worked example. Area: 6 square units.
Example inputs — Side a: 3; Side b: 4; Side c: 5. Starting values are illustrative.
From George Guju, founder of Calculyn · Educational calculator guidance, not professional advice.
How it works
Heron's formula finds the area from the three sides, and the law of cosines finds each angle. Any valid triangle works — right, acute or obtuse.
Assumptions & limitations
- Results are rounded for display.
Formula and assumptions reviewed October 2026 by the Calculyn team. How we build our calculators
Frequently asked questions
What if my sides can't form a triangle?
The two shorter sides must add up to more than the longest side — otherwise no triangle exists.