How do I check my square is actually square?

Everything you cut or fit from a square inherits its error, so an out-of-square square is the one tool fault that quietly ruins whole projects. Cheap squares fail this test at a rate that surprises people, and so do good squares that have been dropped. The test needs no equipment beyond a straight edge and a sharp pencil, and it should be the first thing you do with a new square.

What you'll need

• A genuinely straight reference edge — the factory edge of MDF or plywood
• A sharp pencil or a marking knife
• The square under test

Step by step

1. Find a genuinely straight edge.The factory-cut edge of a sheet of MDF or good plywood is the usual choice — it is machined and it is straight enough. A sawn piece of timber is not, and a board edge you cut yourself is exactly the thing you were hoping to check.
2. Hold the stock hard against that edge.The stock is the thick part. It must be in full contact along its whole length, with no dust or chip under it, and it must stay there while you mark.
3. Scribe a fine line along the blade.Use a marking knife, or a pencil sharpened to a chisel point. A blunt pencil line is a millimetre wide and will hide the very error you are looking for.
4. Flip the square over.Turn it so the stock is now against the same edge but pointing the other way, and the blade lies over the line you just drew. Do not slide it along the edge to a new position and do not turn it upside down onto the other face.
5. Scribe the second line.Set the blade precisely onto the first line at the stock end, then mark along it. You are comparing the square with a mirror image of itself.
6. Read the result at the far end of the blade.One line means the square is true. Two lines that separate as they run away from the edge mean it is out, and the gap between them at the far end is double the real error. Doubling is why this test finds faults you could never see by eye.
7. Decide what to do about a failure.Some engineer's and combination squares can be trued — a combination square's blade slot can sometimes be adjusted, and a cast square can be filed or lapped at the stock. For a cheap pressed-steel try square, the honest answer is to replace it; you will spend more time compensating than a new one costs.
8. Re-test after any drop.A square that hits a concrete floor stock-first is out until proved otherwise. Test it there and then rather than discovering it three joints later.

Tips

• Check the 45 degree face of a combination square the same way, against a corner you have already proved to be 90 degrees, or by marking two 45s from opposite sides and confirming they make a right angle.
• A tiny angular error looks like nothing over a 100mm blade and like a lot over a 600mm cut. That is precisely why the fault survives so long unnoticed.
• Mark the square with a dot of paint once it has passed, so you know which of the several in the drawer is the trustworthy one.
• A large framing or roofing square is checked the same way, but you need a long straight edge — the factory edge of a full sheet is ideal.

Common mistakes

• Testing against an edge you cannot vouch for — The test compares the square with the edge. If the edge is not straight, the result is meaningless in both directions — it can pass a bad square or fail a good one.
• Sliding the square to a different position before the second line — The two marks must share the same starting point. Moving along the edge tests the edge instead of the square.
• Assuming a new square is square — Squareness is not something that is guaranteed by being unused, and pressed steel squares in particular are often out straight from the packet.

Questions people ask