Square check calculator

How this square check calculator works

A square layout check uses the Pythagorean theorem. Enter the two side lengths, and StructCalc calculates the exact diagonal. Measure the diagonal in the real layout and compare the two numbers to see how close the frame, wall, deck or foundation is to square.

This is the same idea as a 3-4-5 triangle, just scaled to the actual dimensions on the job. It is useful for checking plates, batter boards, deck frames, openings and any rectangular layout where a small diagonal error can turn into a visible problem later.

For foundation layout, start by setting the longest sill plate or baseline as your reference. Measure out the shorter side from that line, use the calculator to get the exact diagonal, then pull the opposite corner until the diagonal matches. Once the diagonal is right, the corner is square to your reference line.

Example diagonal check

For a 2400 mm by 3200 mm layout, the expected diagonal is 4000 mm. Mark your two sides from the reference corner, then adjust the free corner until the tape reads 4000 mm diagonally before fastening, bracing or pouring.

Common diagonals

Expected diagonals for layouts proportioned on the 3-4-5 rule. If your two sides match a row, the diagonal below is the number your tape should read.

Square check FAQ

Why does the 3-4-5 rule work?

3² + 4² = 5². Any right triangle with sides in the 3:4:5 ratio has an exact 90° corner, so any multiple — 30-40-50 cm or 3-4-5 m — squares a layout with nothing but a tape measure.

What is the diagonal of a 2400 × 3200 layout?

Exactly 4000 mm: 2400 and 3200 are 3-4-5 sides scaled by 800. The demo computes the same diagonal for any dimensions you enter.

How accurate is a diagonal check?

The math is exact; tolerance comes from tape tension and marking. On large layouts, measure both diagonals — equal diagonals mean the frame is square even if it sits slightly off the planned dimensions.

Should I use bigger numbers than 3-4-5?

Yes when space allows. A longer diagonal amplifies error: a 5 mm miss is far easier to spot on a 5 m diagonal than on a 1 m one. Scale the rule up to the biggest rectangle that fits the site.

Need this on site?

StructCalc for Android includes square check alongside stair, rafter, baluster, raked wall and measurement calculators. It is built for fast field input, clear results and offline use when a browser is not the right tool.