Enter values and select Calculate to see your result.
Calculate triangle angles, area and perimeter from three sides
Enter three positive side lengths in the same unit. Uses the cosine rule and Heron's formula.
How to use the Triangle Calculator
Calculate triangle angles, area and perimeter when all three side lengths are known.
- Enter positive sides a, b and c in one unit.
- Check that the triangle inequality is valid.
- Calculate angles, area and perimeter.
Triangle Calculator formula
Angles use the cosine rule; area uses Heron's formula: √(s(s−a)(s−b)(s−c)).
Worked example
Inputs: Sides 3, 4 and 5.
What to expect: The right triangle has area 6 square units and perimeter 12 units.
How to read your result
If the longest side is not shorter than the other two combined, no triangle exists.
Frequently asked questions
Do I need angles?
No. Three valid side lengths are enough.
What units are used?
Area uses squared input units; perimeter uses the input length unit.