Building Corner Diagonal Check
Compares measured building diagonals and sides with the design rectangle, each against its own tolerance, and states plainly that equal diagonals alone do not prove a rectangle.
Inputs
Design diagonal √(L² + W²), each measured diagonal and side against its own tolerance, and an honest statement of what equal diagonals do and do not prove.
Enter the values and press Calculate.
Or load an example to see a worked result.
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 design length L (corner A to B) and width W (B to C) of the rectangle.
- Add the two diagonals you measured (A–C and B–D) and, for a full check, the four sides — each with the tolerance your project sets.
- Read each check separately: Pending (no tolerance entered), Within your tolerance, or Outside.
How it works
Design diagonal = √(L² + W²). Each measured diagonal is compared with it (signed, measured − design), and the two diagonals with each other (|d1 − d2|), on unrounded values and each against its own tolerance.
Equal diagonals alone do not prove a rectangle — an isosceles trapezoid also has equal diagonals. Only when both pairs of opposite sides AND both diagonals are within tolerance is the figure consistent with a rectangle at that tolerance.
The tool does not tell you which corner to move: diagonal differences alone do not locate the error. Re-fix the corners from control (coordinates or offsets) to find a correction.
Worked example
8 m × 6 m footprint. Design diagonal = √(8² + 6²) = 10.000 m. Diagonals measured at 10.020 and 9.980 m are +20 mm and −20 mm from design and differ by 40 mm — outside a 10 mm tolerance. Two diagonals of exactly 10.000 m with sides of 8.000, 6.300, 7.700 and 6.000 m would still not be a rectangle of the design size.
Tips
- Measure the diagonals and sides with the same tape, tension and temperature so the comparisons are like for like.
Frequently asked questions
Do equal diagonals prove square?
No. A shape with equal diagonals can still have unequal opposite sides (an isosceles trapezoid). Measure all four sides as well; only both together show a rectangle within your tolerance.
Which tolerance is checked?
Each check uses its own: diagonal against design, diagonal against diagonal, and side against design. There is no built-in tolerance — a check with none entered stays Pending.
How far should a corner move?
The diagonals do not say. The same difference can come from different corners moving by different amounts, so re-fix the corners from control rather than shifting one by half the difference.
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.
Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.