How to use the Triangle Calculator
The triangle calculator finds area, perimeter and angles from three side lengths.
Open the Triangle Calculator →
What you need to enter
- Side a
- Side b
- Side c
The formula
Heron's formula: area = √(s(s − a)(s − b)(s − c)) with s = (a + b + c) ÷ 2. Angles come from the law of cosines.
Worked example
Using the default values in the calculator:
- Side a: 3
- Side b: 4
- Side c: 5
| Area | 6 |
| Perimeter | 12 |
| Angle A | 36.87° |
| Angle B | 53.13° |
| Angle C | 90° |
| Type | Scalene |
Tips for better results
- Check the triangle inequality: the two shorter sides must add to more than the longest.
- Use the same unit for all sides.
Common mistakes to avoid
- Using sides that cannot form a triangle.
- Mixing units.