Pythagorean Theorem Calculator
Solve a right triangle from any two sides — find the hypotenuse or a missing leg, plus the area, perimeter and angles.
How to use it
- Fill in any two sides and leave the third box empty.
- c is the hypotenuse — it must be longer than either leg.
- The area, perimeter and one of the angles are worked out too.
Examples
| a = 3, b = 4 | c = 5 |
| a = 5, c = 13 | b = 12 |
The Pythagorean theorem, a² + b² = c², relates the two legs of a right triangle to its hypotenuse. Give the tool any two of the three sides and it solves for the missing one, then works out the area, perimeter and one of the acute angles for free.
Worked example — squaring a foundation
A builder marking out a rectangular foundation 6 m by 8 m checks it's truly square by measuring the diagonal: it should be √(6² + 8²) = √100 = 10 m. If the measured diagonal is off from 10 m, the corners aren't at 90°, and the frame needs adjusting before pouring concrete — the classic 3-4-5 rule scaled up.
Pythagorean triples
| Triple | Scaled example |
|---|---|
| 3-4-5 | 6-8-10, 9-12-15 |
| 5-12-13 | 10-24-26 |
| 8-15-17 | 16-30-34 |
Questions people ask
Does it only work on right triangles?
Yes. For any other triangle you need the law of cosines, which adds a correction term for the angle between the two sides.
What is a Pythagorean triple?
Three whole numbers that satisfy a² + b² = c², like 3-4-5, 5-12-13 and 8-15-17. Handy for setting out square corners on site.
Is anything sent to a server?
No. Every calculation runs in your browser, so nothing you type is uploaded and the page keeps working offline once it has loaded.
Last updated 17 August 2026