Classic tool
Carpentry Square Check (3-4-5)
Check if a piece of furniture is square by measuring sides and diagonal with the 3-4-5 rule.
The Carpentry Square Check applies the 3-4-5 rule (and any multiple of it) to verify if a frame, cabinet or piece of furniture is actually at a 90° angle. For a perfect rectangle, the diagonal must be exactly the square root of the sum of the squares of the two sides.
To use it, enter the two sides of the rectangle in millimetres and the actual diagonal measured with a tape measure. The tool calculates the expected diagonal from the Pythagorean formula, shows the difference in millimetres, the actual angle between the sides (via the law of cosines) and the angular error in degrees, with a visual status: green when square, yellow for small deviations and red when adjustment is needed.
Use the result to check frames, cabinets, drawers and bases before screwing, avoiding the build-up of error that only shows up at the end of the project. Combine this reading with a physical dry check, measuring the diagonal in both directions.
Use clear inputs to get a more useful result.