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Squares and Levels: Setting Out, Checking Square and Reading a Vial
A square is not a ruler with an L on it, and a level is not a bubble in a box. One transfers a right angle to a workpiece and checks that two faces meet at 90 degrees; the other tells you whether a surface is parallel with gravity. Both are references, and a reference that is half a degree out quietly passes that error on to everything built from it. This guide covers the squares and levels in this range and how to use, check and keep them.
What a square is for
A square does three jobs and each one loads the tool differently. Setting out means drawing a line at 90 degrees to a reference edge. Checking means testing a corner or an assembled frame against the blade. Transferring means carrying an angle from a drawing or a sample onto the workpiece. All three depend on the stock, the thick body of the square, sitting dead flat against the edge of the workpiece with no light underneath it.
- Stock. The registering face. Ground and hardened beats a casting that has been painted over.
- Blade. Carries the graduations and must be stiff enough not to bow when a pencil is pressed against it.
- Squareness. The only figure that matters, quoted as mm of error per 100 mm of blade.
- Graduations. Etched and filled markings stay readable for years; printed ones wear away in a tool bag.
Squares in this range
| Square | Best for | Note |
|---|---|---|
| Aluminium try square | Marking and checking timber and sheet | Light, corrosion free, lives in a tool belt |
| Universal measuring square | Setting out with a rise and a run | Sliding head holds a setting for repeat transfer |
| Engineer’s square | Checking machined faces and machine setups | Solid, ground, no graduations to hide the gap |
Try squares for marking and checking
An aluminium try square in 25, 30 and 40 cm sizes with a stainless steel blade is the square that lives on the bench. Aluminium keeps the weight down and the anodised finish shrugs off the damp that comes with green timber, while the stainless blade gives a hard edge for a marking knife. Use the outside face for setting out from an edge and the inside for testing a corner. Never use the blade to lever something apart: a square that has been dropped on its corner has to be checked before it is trusted again.
Universal squares for setting out
A universal measuring square that covers 150 to 300 mm adds a sliding head and extra scales, which is what a job wants when it involves a rise and a run rather than a single right angle. The 150 to 300 mm span covers cabinet work, stair strings and roof setting out, and the adjustable head lets a setting be locked and transferred over and over. Check the head for play before relying on a setting, and treat the scale as a reference while the head does the work of a fence.
How a level works
A spirit level is a curved glass vial set in a machined frame. The bubble floats to the highest point of the curve, and the frame faces are ground parallel to the vial axis, so a bubble sitting between the lines means the face it rests on is horizontal. Sensitivity is quoted in millimetres of movement per metre of run, and a vial that moves 0.5 mm per metre is twice as sensitive as one that moves 1 mm per metre. Length matters as much as sensitivity: a 1000 mm frame shows a small error more clearly than a 300 mm frame because the two ends of the tool move further apart across the same slope.
Magnetic mini levels
Magnetism is what turns a level from a tool you hold into a tool that holds itself. On a steel column, a door frame, a racking upright or a machine bed, a magnetic level stays where it is put while both hands are free to pack, shim or tighten.
Longer frames for setting out
A magnetic mini spirit level from 300 to 1000 mm puts the vial in a slim aluminium frame with magnetic faces. At the long end it will set out a bench, a shelf run or a frame; at the short end it will still go into a cabinet. Pick the frame for the surface, because a 1000 mm level is the right tool for a 3 m bench and the wrong tool for a 300 mm shelf.
Short levels for confined work
A thickened magnetic mini spirit level at 21 mm is the pocket version. It is too short to level a floor, but it fits along a shelf, inside a carcass, across a hinge or on a machine table where a full length frame cannot go. Keep a short level for confined work and a long one for setting out, and remember that any level only measures the surface it happens to be standing on.
Setting out a corner that is square
The 3-4-5 method is the oldest check there is and it still works. Measure 3 units along one edge and 4 units along the other; if the corner is square the diagonal between the two marks is 5 units. Scale it up on site as 600 mm, 800 mm and 1000 mm and use the same tape for all three readings. A second check is better than one: mark both directions against the square, then compare the diagonals of the finished assembly, because two equally wrong corners cancel each other out and any frame with equal diagonals is square whatever the corners look like.
Checking a square or a level
- Square. Hold the stock against a straight edge, mark a line down the blade, flip the square over and mark again from the same point. If the two lines fall on top of one another the square is true, and any gap between them is twice the error.
- Level. Set it on a flat surface, note where the bubble sits, turn the level end for end and read it again. The bubble should sit in the same place relative to the lines. If it moves, the vial needs adjusting or the frame has been twisted.
- Both. Check after any drop, store them hanging or lying flat rather than under heavier tools, and wipe dust off the vial housing, because a speck under the stock is a false reading.
Frequently asked questions
Is an aluminium square accurate enough for furniture?
Yes. A good aluminium try square is more than accurate enough for cabinet and joinery work. Reach for a solid steel engineer’s square when you are checking machined faces or machine setups.
How accurate should a square be?
Look for a stated squareness figure and expect it to hold over the full length of the blade. Check it with the flip test rather than trusting a printed number.
Why does my level read differently when I turn it around?
Either the vial is out of adjustment or the frame is twisted. A level that disagrees with itself cannot be trusted in either direction.
Can I use a magnetic level near a machine?
The magnet is fine, but keep steel swarf off the magnetic face. A chip trapped under the base is a guaranteed false reading.
Which level length should I buy first?
A 600 mm level for general setting out plus a 21 mm mini level for confined work covers most bench jobs. Add a 1000 mm frame when you are levelling long runs.
Where to start
Browse the full Cutting & Measuring Tools range, or the wider Cutting & Measuring Tools category if you also need tape measures, rules, saw blades or gauges. Tell us the job, the material and the tolerance you are working to, and we will point you at the frame length and the square that suit it.
