Law of Sines Calculator
If one side and its opposite angle are known, the sine rule a/sin A = b/sin B gives every missing side or angle. Both parts are below.
A. Missing Side b — when a, A and B are known
B. Missing Angle B — when a, A and b are known
How to use
- Pick Part A to find a missing side, or Part B to find a missing angle.
- In Part A, enter one side, its opposite angle, and a second known angle — all angles in degrees.
- In Part B, enter two sides and one opposite angle to solve for the unknown angle.
- Check both answers when shown: the ambiguous case can give an acute and an obtuse solution.
Note: Sine rule: a/sin A = b/sin B = c/sin C. A solved angle can have two possible answers (acute and obtuse) — the tool shows both when both are possible.
Tips
- The sine rule needs a side-angle pair: one side and the angle opposite it must both be known.
- In Part B, always look for the second possible angle — the tool checks the ambiguous case for you.
- If your angles add up to more than 180°, the inputs are impossible; recheck the known values.