Surveying & site
Diagonal / Square Check
Two tape measurements tell you if a room, deck or extension is truly square - before anything gets fixed down.
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How it works
The diagonal of a right-angled rectangle is sqrt(a2 + b2). Measure both sides and the actual diagonal; the difference is your out-of-square error. The 3-4-5 triangle is the classic special case.
Frequently asked questions
How do I check a room is square with a tape?
Measure both diagonals - in a true rectangle they are identical. Or use this tool: enter the two sides, and compare the predicted diagonal with what you measure.
What is the 3-4-5 rule?
Any triangle with sides 3, 4 and 5 units contains a perfect right angle. Mark 3 units along one wall, 4 along the other - the diagonal should be exactly 5.