Building Corner Diagonal Check
Checks a set-out rectangle for squareness by comparing measured diagonals against tolerance.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the two side lengths of the rectangle or building footprint (L and W).
- Optionally enter the two diagonals you measured on site, and the tolerance you're working to.
- Read the diagonal a true square would have, and whether your set-out is square.
How it works
For a rectangle the diagonal is √(L² + W²), and the two diagonals are equal only when the corners are square. The check compares your measured diagonals with each other and with the ideal value.
If the two diagonals differ by more than your tolerance, the set-out is out of square — nudge one corner along until they match.
Worked example
8 m × 6 m footprint. A square 8 × 6 building has diagonals of √(8² + 6²) = 10.000 m. If you measure 10.02 and 9.98, the 40 mm difference is outside a 10 mm tolerance, so a corner needs adjusting.
Tips
- Equal diagonals guarantee a rectangle only if the opposite sides are also equal — measure all four sides too.
Frequently asked questions
How is this different from the 3-4-5 method?
3-4-5 squares one corner using a right-triangle ratio; the diagonal check proves the whole rectangle at once by comparing both diagonals. They're complementary — use 3-4-5 to set a corner, then the diagonals to confirm the figure.
Is this official course material?
No. It is free study support mapped to surveying course levels — not official North Metropolitan TAFE content or advice. Always follow your lecturer and the official assessment brief, and check your own working.
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Cite this tool
Zerdly. (2026). Building Corner Diagonal Check. Retrieved from https://www.zerdly.com/tools/building-corner-diagonal-check/
Tip: Enter any known values to calculate the remaining results.
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