🔺Triangle Calculator

Enter any 3 sides or angles to solve the rest — SSS, SAS, ASA, AAS, and the ambiguous SSA case are all handled automatically, with a scaled diagram and full working.

Side a

Side b

Side c

Angle A

Angle B

Angle C

Free Triangle Calculator

Given any three sides or angles of a triangle, our free triangle calculator solves the rest instantly. Whether you have three sides (SSS), two sides and an included angle (SAS), two angles and a side (ASA or AAS), or the trickier two-sides-and-a-non-included-angle case (SSA), just enter what you know and get every missing side, angle, the area, and the perimeter — with a scaled diagram and full working shown.

The SSA case is famously ambiguous: depending on your numbers, there can be zero, one, or two valid triangles. Most calculators skip this — ours detects it automatically and shows both solutions when they exist, explaining exactly why.

Invalid inputs (sides that can't form a triangle, angles that don't sum correctly) are caught and explained clearly, not left as a broken result. No sign-up, no downloads — just enter your values and solve. See the About page or the FAQ for more.

Types of triangles (by side length)

Triangles are classified by their side lengths into three main types: equilateral, isosceles, and scalene.

Equilateral triangle — all three sides are equal in length, and all three interior angles are equal, measuring 60° each. Example: a = b = c = 5, A = B = C = 60°.
Isosceles triangle — exactly two sides are equal in length. The angles opposite the equal sides are also equal to each other. Example: a = b = 5, c = 8 gives A = B ≈ 36.9°.
Scalene triangle — all three sides have different lengths, and all three interior angles have different measures. Example: a = 4, b = 5, c = 6.

Formula for a missing side given two sides

If you know two sides and the angle between them (SAS), the third side is found with the Law of Cosines:

c² = a² + b² − 2ab·cos(C)

where C is the angle included between sides a and b. If the triangle is a right triangle (C = 90°), this simplifies to the familiar Pythagorean theorem, since cos(90°) = 0:

c² = a² + b²

If instead you know two sides and a non-included angle (SSA), use the Law of Sines to find the missing angle first, then solve for the remaining side — this is the ambiguous case described above.