Triangle Angle Calculator
Calculate the three interior angles of a triangle from its three side lengths. This page validates the triangle and then uses the law of cosines to find angles A, B, and C.
Formula example:
cos(A) = (b² + c² − a²) / (2bc)
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Privacy: calculations run locally in your browser. No inputs are stored or transmitted.
How it works
After validating the triangle inequality, this calculator applies the law of cosines to compute each interior angle from the side lengths.
The displayed angles are:
A opposite side a,
B opposite side b,
C opposite side c.
Examples
- 3, 4, 5 → angles are approximately 36.87°, 53.13°, and 90°
- 5, 5, 5 → all angles are 60°
FAQ
- What does this triangle angle calculator compute?
It calculates the three interior angles of a triangle from the three side lengths using the law of cosines.
- What formula is used?
This page uses the law of cosines, for example cos(A) = (b² + c² − a²) / (2bc), and similarly for the other angles.
- Why does it validate the triangle inequality?
Because the given side lengths must form a valid triangle before any interior angles can be calculated.
- Do the angles always add to 180°?
Yes, for a valid Euclidean triangle the interior angles sum to 180 degrees, aside from tiny rounding differences in display.
- Are calculations stored?
No. Everything runs locally in your browser.