Math

Triangle Calculator (Three Sides)

Enter three sides and get everything else.

By TheCalcTools Updated October 17, 2026 Formula checked and tested

Your numbers

Side a

Side b

Side c

Area

—

Perimeter

—

Angle A (°)

—

Angle B (°)

—

Angle C (°)

—

Type

—

Enter the three side lengths in any unit.

Heron’s formula gives the area from the semi-perimeter s.

The law of cosines gives each angle.

s = (a + b + c) ÷ 2Area = √(s(s − a)(s − b)(s − c))

Using the default values shown in the calculator:

Result:

What is the triangle inequality?

Each side must be shorter than the sum of the other two; otherwise the sides cannot close into a triangle.

Is the triangle right-angled?

If a² + b² = c² for the longest side, one angle is exactly 90°. Check the angle results.