Check whether a layout is square using the 3-4-5 method and diagonal measurements for foundations, decks and walls.
A triangle with sides in the ratio 3:4:5 always has a perfect right angle. Measure 3 units along one edge and 4 along the other; if the diagonal between those points is exactly 5 units, the corner is square. Any multiple works — 600–800–1000mm is handy on site.
For a rectangle, the quickest check is that both diagonals are equal. If they match the calculated value, your layout is square.
If a triangle has sides of 3 and 4 units meeting at a corner, the diagonal will be 5 units when that corner is a perfect right angle. Any multiple, such as 600-800-1000mm, works the same way.
Measure both diagonals. If they are equal, and match the diagonal this calculator gives for your width and length, the layout is square.
Any units you like, as long as the width and length use the same one. The diagonal comes out in that same unit.
For most building and landscaping work, within a few millimetres is fine. This tool flags anything more than about 0.5 percent off as out of square.
Square foundations, decks, walls and tiling save time and material later. Out-of-square layouts lead to tapered gaps, awkward cuts and structural problems.
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