Diagonal calculator

Enter the length and width for the diagonal a square frame, deck or slab should measure. Enter the two diagonals you measured to see how far out of square it is and which corners to push.

Square-up calculator

Millimetres. The same maths as Check Square in the MeisterCalc app.

This calculator needs JavaScript, which is turned off or didn't load. You can still do it by hand: the formula, the worked examples and the tables below give the same answers.

Published 6 November 2026 · By Sai Nath

Worked example: a 4800 × 3600 mm frame

Target diagonal: √(4800² + 3600²) = √36,000,000 = 6000 mm.
The tape reads 6012 mm one way and 5988 mm the other: out of square by 24 mm.
Push the two corners at the ends of the 6012 mm diagonal towards each other. It comes in by half the gap, 12 mm, and the other grows, until both read about 6000 mm.

Push a little, then measure both diagonals again. If the short one becomes the long one, you have gone past square.

How it works

Target diagonal = √(length² + width²), the long side of a right triangle.
Is it square? takes both measured diagonals. The difference is how far the frame is racked; the longer diagonal is the one to shorten.
The calculator, like the app, calls a frame square when the diagonals are less than 1 mm apart.

This web calculator takes millimetres. Multiply inches by 25.4 to convert.

Check Square in MeisterCalc

Check Square in the MeisterCalc app

Open Home ▸ Check Square ▸ Target Diagonal and enter Length 4800 mm and Width 3600 mm: Diagonal 6000.0 mm, with the frame drawn. Is It Square? takes the two measured diagonals and shows how far out the frame is. The app also works in feet, inches and sixteenths.

On Home, tap Check Square
Enter 4800 × 3600 mm: read 6000.0 mm

The app version, with every input, result and screen: Check Square in MeisterCalc.

Check Square is one of the three free calculators in the app, with Concrete, Stairs and Check Square: no trial and no account. The others unlock with a one-time purchase, no subscription required, or with a weekly subscription that starts with a 3-day free trial. Every calculation runs on the phone, so it works with no signal on site. It runs on Android 7.0 or later. Get MeisterCalc on Google Play

Questions

What is the 3-4-5 rule?

A triangle with sides of 3, 4 and 5 units has a right angle. Measure 3000 mm along one side and 4000 mm along the other: the diagonal between the marks should read 5000 mm. The diagonal calculator is the same check for any size.

How close is close enough?

The tolerance for a job comes from the drawings, the specification or the builder you work for. A few millimetres matter more on a small frame than a big one.

What about an L-shaped slab?

Split it into rectangles and square each one.

Sources

Pythagorean theorem (Wikipedia)

Building rules are set by the National Construction Code, the NZ Building Code or the Approved Documents and by your council, not by this page or the app.