Triangle Calculator
Solve any triangle: right triangles, three sides, or two sides and an angle.
All calculations happen locally in your browser — nothing is uploaded, stored or tracked.
Pick the known data
Right triangle (two legs), SSS, or SAS.
Enter the values
Sides, or sides and the included angle.
Read the solution
All angles, the missing side, area, perimeter and height.
What Is a Triangle Calculator?
A triangle calculator solves for everything about a triangle from a minimum set of known values: angles, side lengths, area, perimeter and height. Three modes cover the common cases: a right triangle with two legs known, a triangle with all three sides known (SSS), and a triangle with two sides and the included angle known (SAS).
The Methods
SSS triangles use the law of cosines to recover each angle, then Heron's formula for the area. SAS triangles use the law of cosines for the missing side and the law of sines for the remaining angles.
Validity Checks
The calculator rejects impossible triangles: in SSS mode the three sides must satisfy the triangle inequality (each side shorter than the sum of the other two), and angles must be between 0 and 180 degrees.
Your data never leaves your device.
Every calculation on this site runs in your browser using vanilla JavaScript. Nothing is uploaded, stored or tracked.
- 100% local, client-side calculations
- No data stored on any server
- No tracking, no analytics, no ads scripts
- Works offline once loaded
Frequently Asked Questions
How do I find the missing side of a right triangle?
Use the Pythagorean theorem: the hypotenuse squared equals the sum of the squares of the two legs.
How do I calculate triangle angles from three sides?
Use the law of cosines: cos(C) = (a² + b² − c²) / (2ab), and likewise for the other angles.
How do I find the area of a triangle with three sides?
Use Heron's formula with the semi-perimeter. The calculator applies it automatically in SSS mode.
What is the SAS rule?
Side-Angle-Side: when you know two sides and the angle between them, the law of cosines gives the third side.
What if my triangle is impossible?
The calculator validates inputs and shows an error — for example when three sides violate the triangle inequality.