Enter all three side lengths (SSS) to solve the triangle's angles, area and perimeter.
Three side lengths fully determine a triangle's shape and size (the SSS case) — there's exactly one triangle with those three sides. The Law of Cosines then recovers all three angles from the sides alone, so no angle input is needed.
The triangle inequality requires each side to be shorter than the sum of the other two (e.g. a < b + c). If any side is too long relative to the others, the three segments simply can't close into a triangle, so the calculator flags it instead of showing an invalid result.
These classify the same triangle two ways: by sides (Equilateral = all equal, Isosceles = two equal, Scalene = all different) and by angles (Acute = all angles under 90°, Right = one angle is 90°, Obtuse = one angle over 90°).