The 3-4-5 rule, scaled to any size
A corner is square when its diagonal equals √(A² + B²) — the Pythagorean theorem applied to decking, foundations, fences and tiling. The classic 3-4-5 triangle scales up: 6-8 gives a 10-foot diagonal, 30-40 a 50-footer. Foundation crews stretch strings and check both diagonals of the rectangle.
Check both diagonals
One matching diagonal only proves the shape is a parallelogram — a rhombus has equal sides and unequal angles. Measure both diagonals: when they're equal to each other (and to the calculated length), the rectangle is truly square. Tape measure, two people, five minutes — this is the highest-leverage check in construction.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- American Wood Council — span tables, lumber design values and the Wood Frame Construction Manual.
- APA — The Engineered Wood Association — panel span ratings and sheathing installation guidance.
Figures, formulas and reference tables last verified: 19 September 2026.