Triangle Perimeter Calculator

What Is Triangle Perimeter?

The perimeter of a triangle is the sum of its three side lengths: P = a + b + c.

If you know all three sides, calculation is straightforward. When you only have partial information — two sides and an angle, or one side and two angles — trigonometry and the law of sines/cosines fill in the missing sides.

Perimeter Formulas

From three sides: P = a + b + c.

From two sides (a, b) and the included angle (C): Use the law of cosines to find side c: c² = a² + b² - 2ab x cos(C). Then P = a + b + c.

From one side (a) and two angles (B, C): Find the third angle: A = 180° - B - C. Then use the law of sines to find b and c: b = a x sin(B)/sin(A), c = a x sin(C)/sin(A). Then P = a + b + c.

Types of Triangles

By sides: Equilateral (3 equal sides), Isosceles (2 equal sides), Scalene (no equal sides).

By angles: Acute (all angles < 90°), Right (one 90° angle), Obtuse (one angle > 90°).

Perimeter formula is the same regardless of triangle type — always the sum of all three sides.

Applications

Construction: Calculate fencing length for triangular plots; determine trim or edging material length.

Education: Solve geometry problems involving triangle measurements.

Land surveying: Compute boundary lengths for triangular land parcels.

Design: Frame dimensions for triangular architectural elements or art projects.