About this calculator
The result also names the triangle type.
How to use it
- Choose sides or angles.
- Enter the values.
- Read the angles.
The formula
cos A = (b² + c² − a²) ÷ 2bc; A + B + C = 180°
- A
- angle opposite side a
Worked example
Sides 7, 8 and 9
- A ≈ 48.19°, B ≈ 58.41°, C ≈ 73.40°.
- All acute, so an acute scalene triangle.
What the result means
The largest angle is opposite the longest side.
Assumptions
- All lengths in the same unit.
Limitations
- Results are rounded to 10 significant figures.
Frequently asked questions
What do the angles of a triangle add up to?
180°.
How do I find angles from sides?
Use the law of cosines.
What is an obtuse triangle?
One with an angle above 90°.
Last reviewed on 3 October 2026. Found a mistake? Tell us.