What is this right triangle calculator?
This calculator uses the Pythagorean theorem (sideA² + sideB² = hypotenuse²) to instantly solve for the missing side of a right triangle from any two known values. Enter Side A and Side B and it solves for the hypotenuse (diagonal); enter the hypotenuse and one side and it solves for the other side. The classic "3-4-5 rule" used in carpentry, framing, landscaping, and deck building works perfectly here too, but you're not limited to it — plug in whatever real measurements you have. Change any value and the diagram below redraws to match your actual ratio.
How to use it
Enter any two of Side A, Side B, and the Hypotenuse — the remaining field fills in automatically. If all three are already filled and you enter a new value somewhere, the field you filled in longest ago gets recalculated and updated instead. Mark out the two sides on-site using the calculated hypotenuse, measure the actual diagonal, and enter it in the "On-site verification" field to see how far off it is from the ideal and whether the corner is square.
When it comes in handy
- Quickly checking whether a wall or floor corner is square without a framing square or protractor
- Squaring up baselines for deck, tile, or landscaping projects
- Working out a diagonal length in advance for furniture or woodworking frames
- Quickly back-calculating a missing side from two measurements you already have
Frequently Asked Questions
- Do I have to use a 3:4:5 ratio?
- No. Any two values that form a right triangle will work — 3:4:5 is just the most well-known example, and this calculator works for any ratio.
- What happens if I enter a new value when all three are already filled?
- Type into any of the three fields and the field you filled in longest ago gets recalculated and updated automatically.
- What's the tolerance for the on-site verification check?
- A difference within about 0.5% of the ideal hypotenuse length is treated as "very close to a true right angle" — a margin that accounts for typical tape-measure accuracy on a job site.