What should the diagonal be to square up a frame, and which way do I push?
Published · Updated · By Sai Nath
A square frame's diagonal is √(length² + width²), so 4800 × 3600 mm should measure 6000 mm corner to corner. If your two diagonals differ, the frame is racked: push the two corners at the ends of the longer diagonal towards each other until both read the same. The long one comes in by half the difference.
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.
How does the square-up calculator work?
It uses Pythagoras for the target and a subtraction for the check, exactly as MeisterCalc's Check Square does.
- Target diagonal = √(length² + width²). A rectangle's diagonal is the long side of a right-angled triangle, so this is what both diagonals measure when the corners are at 90°.
- Racking gap = the longer diagonal − the shorter one. Zero means square. The calculator calls a frame square when the gap is under 1 mm, the same test the app uses.
- How far to push: the long diagonal comes in by half the gap, and the short one grows by about the same, so they meet in the middle. In a parallelogram the squares of the two diagonals always add up to 2 × (length² + width²), which is why both settle at almost exactly the target.
Worked example: what should a 4800 × 3600 mm frame measure?
- Square the sides: 4800² = 23,040,000 and 3600² = 12,960,000.
- Add them: 36,000,000.
- Take the square root: √36,000,000 = 6000 mm.
Both diagonals should read 6000 mm. This frame is in the 3:4:5 ratio (3600 : 4800 : 6000), so the whole frame is a 3-4-5 triangle, and the diagonal comes out round. Most frames don't: 2400 × 1200 mm, a full sheet, should measure 2683.3 mm.
Worked example: the diagonals read 6012 and 5988 mm
Same 4800 × 3600 mm frame, but the tape says 6012 mm one way and 5988 mm the other.
- The racking gap is 6012 − 5988 = 24 mm, so the frame is out of square.
- Diagonal 1 (6012 mm) is the longer one. Push the two corners at its ends towards each other.
- It needs to come in by half the gap, 12 mm, to 6000 mm. Diagonal 2 grows from 5988 to about 6000 mm as you push.
- 6000 mm is also the target for a 4800 × 3600 frame, so the readings and the sides agree.
Push a little, then measure both diagonals again. If the short one becomes the long one, you've gone past square: push back by half the new difference.
Which corners do I push?
The two corners at the ends of the longer diagonal, towards each other. It's the same move as pulling the ends of the shorter diagonal apart; use whichever is easier on the job. A sash clamp or a ratchet strap across the long diagonal does it in small, controlled steps on a wall frame or a deck frame. Fix a temporary brace once both diagonals match, before the frame moves again.
In the MeisterCalc app, Check Square's Is It Square? mode shows half the gap as Push each corner by. Read it as how far the long diagonal comes in, as this page does, and stop when both diagonals read the same.
3-4-5 multiples in millimetres
Before there's a frame to measure, the 3-4-5 rule sets out one right angle: mark 3 units along one line and 4 along the other, then move the second line until the marks are 5 units apart. The bigger the triangle, the smaller the error from a millimetre of misreading, so use the largest that fits. The calculator's Target Diagonal mode works it out in 100 mm steps.
| Along one side (3) | Along the other (4) | Across (5) |
|---|---|---|
| 300 mm | 400 mm | 500 mm |
| 600 mm | 800 mm | 1000 mm |
| 900 mm | 1200 mm | 1500 mm |
| 1200 mm | 1600 mm | 2000 mm |
| 1800 mm | 2400 mm | 3000 mm |
| 2400 mm | 3200 mm | 4000 mm |
| 3000 mm | 4000 mm | 5000 mm |
| 3600 mm | 4800 mm | 6000 mm |
3-4-5 squares one corner. Once all four sides are in, check the diagonals: they check all four corners at once. There's more on the method in how to use the 3-4-5 rule.
Target diagonals for common frame sizes
| Frame (length × width) | Diagonal when square | Largest 3-4-5 that fits |
|---|---|---|
| 1200 × 900 mm | 1500.0 mm | 900 × 1200 × 1500 |
| 2400 × 1200 mm | 2683.3 mm | 1200 × 1600 × 2000 |
| 3000 × 2400 mm | 3841.9 mm | 2100 × 2800 × 3500 |
| 3600 × 2400 mm | 4326.7 mm | 2400 × 3200 × 4000 |
| 4000 × 3000 mm | 5000.0 mm | 3000 × 4000 × 5000 |
| 4800 × 3600 mm | 6000.0 mm | 3600 × 4800 × 6000 |
| 6000 × 4000 mm | 7211.1 mm | 3900 × 5200 × 6500 |
How the last column is found, for 3000 × 2400 mm: the 4 side must fit along the 3000 mm side and the 3 side along the 2400 mm side, so one unit is at most 3000 ÷ 4 = 750 mm and 2400 ÷ 3 = 800 mm. The smaller wins, and in whole 100 mm steps that is 700 mm: 3 × 700 = 2100, 4 × 700 = 2800 and 5 × 700 = 3500 mm.
For the maths behind the first column, see how to calculate the diagonal of a rectangle.
Common mistakes when squaring up
- Different reference points. Outside corner on one diagonal and inside corner on the other gives a gap that isn't there. Use the same point at all four corners.
- Skipping the sides. Equal diagonals prove 90° corners only when opposite sides are equal. A frame with one long side can have equal diagonals and still be out. Check the sides first.
- A sagging or bent tape. On a long diagonal, a tape that isn't straight and tight reads long. Pull it tight, keep it clear of the frame, and use a laser measure on big set-outs if you have one.
- Pushing the wrong pair of corners. Pushing the ends of the short diagonal together makes things worse. It's the long one that comes in.
- Moving each corner by half the gap. Half the gap is how far the long diagonal shortens in total, not a distance for each corner. Push, measure, repeat.
- Trusting one push. Frames spring back. Measure both diagonals again after each push and before you brace or fix down.
Questions people ask
How close is close enough?
This calculator and the app call a frame square when the diagonals are less than 1 mm apart. The tolerance for a particular job comes from the drawings, the specification or the builder you're working for. The same few millimetres matter more on a small frame than a big one.
Does it work in feet and inches?
This web calculator takes millimetres only. Multiply inches by 25.4 to convert (16 ft = 4876.8 mm). The MeisterCalc app works in feet, inches and sixteenths too: Settings ▸ Units ▸ Default units ▸ Imperial (ft/in).
What about an L-shaped slab or a building with more than four corners?
Split it into rectangles and square each one. Setting out a whole building from string lines is covered in how to set out a building or slab square.
What this calculator doesn't do
- It doesn't measure anything. The answer is only as good as your two tape or laser readings.
- It's for rectangles. It can't tell you a side is the wrong length; it assumes the length and width you enter are what's on the ground.
- It checks square, not level or plumb.
- Half the gap is a guide to how far to push. Measuring both diagonals again is the real test.
- Millimetres only, here. Tolerances come from the job, not from this page.
For the full tape-measure method, step by step, read how to check if a frame is square.
Check Square in MeisterCalc
The same two modes, Target Diagonal and Is It Square?, are in MeisterCalc's Check Square calculator, with a live diagram of the frame: Home ▸ Check Square. It's one of three MeisterCalc calculators that are free to use, along with Stairs and Concrete, and it works offline on site, on Android 7.0 or later. The other 12 calculators unlock with a weekly subscription that starts with a 3-day free trial, or with a one-time lifetime unlock; Google Play shows the prices in your currency. There's no iPhone version yet.