3-4-5 square checker
Check a corner is square with nothing but a tape measure.
Legs & measured diagonal
Distance measured along one side from the corner.
Distance measured along the other side from the corner.
Distance between the two far ends. Expected if square: 5000mm.
Corner check
— within a 3mm site tolerance of a true right angle.
Only as good as the tape. Both legs must be measured from the same true corner point, and the tape kept straight and taut — a sagging tape will read a corner as more out of square than it really is.
The 3-4-5 method is one of the oldest tricks on a building site, and it still works because it's just Pythagoras with easy numbers: any right angle scaled so its two legs sit in a 3:4 ratio has a diagonal in a 5 ratio. Measure two legs out from a corner, measure the straight line between their far ends, and compare it to what the legs say it should be — the gap tells you exactly how far out of square the corner is, and which way to nudge it.
How to take the measurements
From the corner you're checking, measure a set distance along one side (leg A) and mark it, then measure a set distance along the other side (leg B) and mark that too — they don't have to be exactly 3m and 4m, any two known lengths work, but round numbers make the marks easier to find again. Then run the tape between the two marks and read the diagonal. Enter all three figures and this tool works out what that diagonal should be if the corner were a true 90°, and how far your reading is off. A positive error means the diagonal came in longer than expected — the corner is opening up past square, so pull it in. A negative error means the diagonal came in short — the corner is pinched tighter than square, so push it out.
Keep the tape straight and taut for both legs and the diagonal — a sagging or kinked tape reads longer than the true straight-line distance and will make a square corner look out when it isn't.
A worked example
The classic case: leg A of 3000mm and leg B of 4000mm should give a diagonal of exactly 5000mm if the corner is square (3² + 4² = 5², or 3,000,000 + 4,000,000 = 5,000,000 in mm terms). Measure the diagonal on site and get 5015mm instead, and you're 15mm out — well past a workable tolerance for setting out a slab or a stud wall, and the corner needs pulling in before you go any further. Get 4996mm and you're within a few millimetres, close enough to call it square for most site work.
What this isn't
This only checks the one corner you measured — a building can be square at one corner and out at the others, so it's worth checking more than one if the whole structure needs to be true. It also assumes the tape measure itself is accurate and that both legs were genuinely measured from the same corner point, not from slightly different spots along the wall. A cheap or stretched tape, or a leg measured from the wrong end, will throw the whole check off even though the arithmetic behind it is exact.
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