Triangle Calculator

Calculate area, perimeter, and angles of any triangle from sides or angles.

Enter any 3 known values (at least one must be a side).

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How it works

Enter any combination of sides (a, b, c) and angles (A, B, C) that fully defines a triangle. The tool applies the Law of Cosines to find missing sides and the Law of Sines to find missing angles. It then computes the perimeter, area (via Heron's formula), and classifies the triangle by sides (scalene, isosceles, equilateral) and by angles (acute, right, obtuse). All computation runs locally in your browser.


              Area = √(s(s−a)(s−b)(s−c)), s = (a+b+c)/2 (Heron's formula)

Common use cases

  • Finding the area of a triangular plot of land or roof section.
  • Solving geometry homework problems involving missing sides or angles.
  • Checking that three given sides form a valid triangle before cutting material.

Frequently asked questions

What is the minimum input needed?

You need at least three pieces of information that uniquely determine the triangle: three sides (SSS), two sides and the included angle (SAS), two angles and a side (ASA or AAS), or a right angle with two sides (right triangle).

What if my inputs do not form a valid triangle?

The tool will show an error. Common invalid cases: the sum of any two sides must exceed the third side, and all angles must sum to 180°.