Triangle Side Calculator

Calculate missing triangle side lengths. Free online triangle side finder calculator.

The Triangle Side Calculator finds missing side lengths using various methods depending on what's known: Pythagorean theorem for right triangles, Law of Cosines for SAS, or Law of Sines for AAS/ASA cases. Enter your known measurements and this versatile tool determines the appropriate method to find missing sides.

Common use cases

  • Construction measurements
  • Land surveying
  • Geometry problem solving
  • Design and architecture
  • Navigation calculations

How to use

  1. Select what information you have
  2. Enter known sides and/or angles
  3. Indicate if it's a right triangle
  4. Click calculate for missing side(s)
  5. View the method used

FAQ

What information do I need?

At least 3 pieces of information (sides/angles), with at least one side. Three angles isn't enough—infinite similar triangles exist.

Which method is used?

Right triangle: Pythagorean. SAS/SSS: Law of Cosines. AAS/ASA: Law of Sines.

Can any three lengths form a triangle?

No, triangle inequality requires each side < sum of other two sides.

What about isosceles or equilateral?

These have special properties (equal sides) that simplify calculations.