Triangle Calculator

Find all three angles and the area of a triangle from its three side lengths (SSS).

⏱ Updated: 17 Sep 2026

Calculator

Free triangle calculator: find all three angles and the area of a triangle from its three side lengths.

Angle A
°
Angle B
°
Angle C
°
Area
Perimeter

Enter the three side lengths of a triangle to find its angles, area, and perimeter.

How to Use the Triangle Calculator

All three side lengths go in; angles, area, and perimeter all come out together, no need to compute them one at a time.

Three sides, no angles measured, and you can still find every angle and the full area. This Triangle Calculator solves from side lengths alone (SSS), leaning on two formulas that don't get taught together as often as they should.

angle A = arccos((b² + c² − a²) ÷ (2bc))
s = (a + b + c) ÷ 2
area = √(s(s−a)(s−b)(s−c))

The first is the Law of Cosines, rearranged to solve for an angle instead of a side — rotate the same formula for angles B and C, or just subtract the two you've found from 180°. The second is Heron's formula, which gets you area with zero need for a separate height measurement — s is just the semi-perimeter, half the total perimeter.

Not every three lengths cooperate, though. Try 2, 3, and 10 and there's no triangle to solve — the two shorter sides simply can't stretch far enough to close the shape, which is the triangle inequality in practice: each side has to be shorter than the sum of the other two.