Squaring Corners 3-4-5 Calculator
To check that a corner is square, measure 3 units along one side, 4 units along the other, and the diagonal between those two points. This works out what that diagonal should be.
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How it works
Add the squares of the two sides, then take the square root. That gives the diagonal a true right-angle corner has. This is the Pythagorean theorem.
Any multiple of 3, 4 and 5 works, such as 6, 8 and 10, or 9, 12 and 15. A bigger triangle shows small errors more clearly over a wide wall.
Worked example
With sides of 3 ft and 4 ft, the diagonal should be 5 ft. Your measured diagonal is 5 ft, a difference of 0 in.
Questions
Why use 3, 4 and 5?
Those numbers make a right triangle: 3 squared plus 4 squared is 25, which is 5 squared. They are easy to measure on a job.
Which points do I measure between?
Measure out from the corner point along each side, then measure straight across from the end of one side to the end of the other.
What if my numbers are a little off?
Small differences come from the tape and the marks. Measure again on a bigger triangle, such as 6, 8 and 10, to see the result more clearly.
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