Check Square Calculator
Find the diagonal of a rectangle to check that a foundation, deck or frame is square, using the 3-4-5 method.
Enter your values and press Calculate.
How to use the Check Square calculator
For a quick check on any corner, measure 3 ft one way and 4 ft the other — the diagonal should be exactly 5 ft.
Formula
Diagonal = √(A² + B²). If both diagonals of a rectangle match, it is square.
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