How to check a try square for accuracy
Updated: October 2026
You have unpacked a new square, or dropped an old one, and now two pieces cut from its lines no longer meet cleanly. Knowing how to check a try square takes one straight reference edge, a fine marker and a simple flip test. A star rating cannot answer the same question. The [try square comparison](/hub/measuring-and-marking-tools/try-square) shows the five amazon.co.uk listings and the limits of their review evidence. Before using any of them for a project, draw one line, reverse the tool and compare the second line. If the joint moves or the lines open into a V, stop using that square as a 90° reference.
You have unpacked a new square, or dropped an old one, and now two pieces cut from its lines no longer meet cleanly.
You have unpacked a new square, or dropped an old one, and now two pieces cut from its lines no longer meet cleanly. Knowing how to check a try square takes one straight reference edge, a fine marker and a simple flip test. A star rating cannot answer the same question. The try square comparison shows the five amazon.co.uk listings and the limits of their review evidence. Before using any of them for a project, draw one line, reverse the tool and compare the second line. If the joint moves or the lines open into a V, stop using that square as a 90° reference.
Our top picks
| Badge | Product | Joint | Stated length | Check first | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  WORKPRO WORKPRO 200 mm try square | Three pins | 200 mm | Sharp edge in reviews | 9.2 | about £11.99 | View offer |
| Preis-Tipp |  S & S PRODUCTS S & S 150 mm try square | Punch indents | 150 mm | Loose joint in reviews | 8.2 | about £4.49 | View offer |
| Alternative |  Prexiso PREXISO 250 mm try square | Not stated | 250 mm | 15 reviews only | 8.7 | about £16.99 | View offer |
On this page
Draw, flip and compare
Start with a flat board that has one straight, clean edge. Place the stock firmly against that edge, keep the blade flat, and draw a fine line with a sharp pencil or a knife suited to the blade material.
Flip the stock through 180° so it sits on the other side of the line, still against the same edge, and draw a second line from the same starting area. Keep the same marker. If the lines overlap or stay parallel, the square agrees with itself. If they open into a V, reversing the square has doubled the error, so the tool is not giving the same 90° in both orientations.
A visible gap over the blade length means a cut from one orientation will not agree with a cut from the other. Repeat once on another straight edge if the result is surprising.
Check whether the board or marker caused the failure
A flip test depends on a straight reference edge. If that edge bows, the stock sits differently after reversal and a sound square can look wrong. Sight along the board, check it with a trusted straightedge, and clear dust or a splinter before you blame the tool.
A blunt or tilted pencil moves the apparent line. A marking knife is finer, but the edge of the square must tolerate it. One PREXISO review warns that a utility knife can cut an aluminium edge, so use its scriber or a sharp pencil. Pushing hard on a long blade can flex it or twist a loose joint. If the result changes with pressure, inspect the joint. Once the setup is repeatable, diverging lines point back to the square. After it passes, see how to use a try square.
Stop when the joint moves
A blade and stock that move relative to each other cannot stay a dependable square. Hold the tool by the stock and check for play. Do not bend the blade to hunt for movement. Any click or shift is enough to stop.
One Gunpla owner says the rivet holding 90° came off in use. S & S reviews describe punch indents, looseness, factory glue and an owner adding more adhesive. They show what to inspect on arrival.
Presch reviewers are mixed: some passed a flip test, others returned a square that was not square. The file mixes sizes, so 1,266 reviews cannot predict your 300 mm sample. WORKPRO's stronger rating is not an accuracy grade. Axminster warns that a square dropped onto concrete may no longer be 90°. Re-test after a fall.
Deal with a faulty square promptly
Do not default to filing or bending a new riveted square back to 90°. Altering it can make a return harder and leave an unstable joint. If a new product fails a careful flip test, photograph the result and use the retailer's faulty-goods process.
Section 22 of the Consumer Rights Act 2015 sets out a short-term right to reject that normally runs for 30 days. This is general consumer information, not a promise of an automatic Amazon refund. One Presch reviewer said the return date had passed before the error was spotted, so test before you cut timber.
The best try square comparison shows where review files are thin or mixed. It cannot replace checking the unit in the box. None of these five listings claims BS 939 or BS 3322.
Common questions
What do diverging lines mean in a flip test?
They mean the square does not give the same line after reversal, assuming the reference edge is straight and your marking technique is consistent. The flip doubles the visible angular error. Repeat once on a verified edge. If the V remains, stop using the tool as a 90° reference.
Can a bowed board make a good square look wrong?
Yes. The stock registers differently against a bowed or damaged edge after it is flipped. Check the board with a trusted straightedge, remove debris and use a flat face. A square can only be assessed against a reference that is itself straight.
Does a high star rating prove accuracy?
No. Stars combine many aspects of ownership and do not measure the angle of your sample. WORKPRO's 4.7 stars from 652 reviews and Presch's 1,266-review file provide shopping context. Neither is a calibration certificate. Flip-test every new square.
What should you do if a rivet moves?
Stop using the square for 90° marking. A moving joint changes the relationship between blade and stock, so a good result now may not persist. For a new purchase, document the movement and contact the retailer rather than relying on glue or repeated adjustment.
Can you file a cheap try square true?
Experienced toolmakers sometimes true particular square designs, but that is not a suitable default for these riveted products. Filing may address one angle while leaving a loose joint or a damaged edge. Return a faulty new tool. Retire or professionally assess an old one when dependable accuracy matters.
When do you need a graded engineer's square?
Use one when machining, inspection or repeatable workshop setup requires a declared accuracy grade. BS 939 is not claimed by any product in this comparison. A checked carpenter's try square may be adequate for ordinary timber marking. It should not be presented as graded metrology equipment.
Related pages
Sources
Public UK sources for this comparison. Prices, stars and review counts are the amazon.co.uk figures in the listing files, not a live reload.
- BSI, BS 3322:1981 — carpenters' squares and bevels (retrieved 16 October 2026)
- BSI, BS 939:2007 — engineers' squares (retrieved 16 October 2026)
- Wonkee Donkee Tools — engineer's square accuracy (retrieved 16 October 2026)
- Wonkee Donkee Tools — how to use a try and mitre square (retrieved 16 October 2026)
- Wood Workers Workshop — flip test (retrieved 16 October 2026)
- Axminster — measuring and marking (retrieved 16 October 2026)
- Axminster — types of woodworking square (retrieved 16 October 2026)
- Consumer Rights Act 2015, section 22 (retrieved 16 October 2026)
- Faithfull — carpenters' try squares (retrieved 16 October 2026)