Find a diagonal or check a layout for square using the Pythagorean theorem. Enter two sides; add your measured diagonal to test whether a corner is truly 90 degrees.
Diagonal
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Hypotenuse of the two sides.Usage Tip
For big jobs use multiples of 3-4-5 (like 6-8-10) for more accuracy. Adjust one corner until the two diagonals match exactly before you fasten anything.
THE MATH
Diagonal c = √(a² + b²)
A corner is square when the measured diagonal equals c
3-4-5 rule: sides of 3 and 4 give a 5 diagonal
A corner is square when the measured diagonal equals c
3-4-5 rule: sides of 3 and 4 give a 5 diagonal
The Pythagorean theorem: in a right triangle the diagonal (hypotenuse) squared equals the sum of the other two sides squared.
To check a rectangle for square, its two diagonals must be equal; this gives the target length.
To check a rectangle for square, its two diagonals must be equal; this gives the target length.
To square a layout, measure both diagonals of the rectangle — they are equal only when all four corners are 90°.
The 3-4-5 method scales: 6-8-10 or 9-12-15 work the same and are easier on big layouts.
Small errors compound over distance; re-check square after fastening, since nailing can shift things.
The 3-4-5 method scales: 6-8-10 or 9-12-15 work the same and are easier on big layouts.
Small errors compound over distance; re-check square after fastening, since nailing can shift things.