How to use a speed square
Updated: 22 October 2026
You are marking a board end in a UK garage. The line has to be 90° to a straight edge, or 45° for a mitre. How to use a speed square starts with the lip on that edge. This page is the cross-cut, the scribe, a short saw fence, and a flip test. Prices and the twelve listings are on the [speed square comparison](/hub/measuring-and-marking-tools/speed-square). Peter F. wrote this on 21 October 2026 from those listings and the public sources named below. No laboratory test. A qualifying purchase can earn us a commission at no extra cost to you.
You are marking a board end in a UK garage. The line has to be 90° to a straight edge, or 45° for a mitre. How to use a speed square starts with the lip on that edge.
You are marking a board end in a UK garage. The line has to be 90° to a straight edge, or 45° for a mitre. How to use a speed square starts with the lip on that edge. This page is the cross-cut, the scribe, a short saw fence, and a flip test. Prices and the twelve listings are on the speed square comparison. Peter F. wrote this on 21 October 2026 from those listings and the public sources named below. No laboratory test. A qualifying purchase can earn us a commission at no extra cost to you.
Our top picks
| Badge | Product | Fence | Size | Drill gauge | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Preciva Preciva speed square, black | T-shaped base | 180 mm | 3–15 mm | 8.6 | about £8.49 | View offer |
| Alternative |  DeWalt DeWalt speed square, 18 cm | Fixed large base | 180 mm | n/a | 7.2 | about £16.25 | View offer |
| Best value |  Difistik Difistik speed square, 7 inch | T-shaped base | 178 mm | 3–15 mm | 7.4 | about £6.99 | View offer |
On this page
Hook the lip, then mark 90° or 45°
Press the lip, the T-shaped fence, flat against a straight edge. For a speed square 90 degree line, draw along the leg that meets the lip at a right angle. You are copying the corner of the triangle onto the face. A bowed edge produces a line that is square to the bow, so sight the edge if the board has been leaning in a damp shed.
A speed square 45 degree line uses the same lip and the hypotenuse. Swanson’s reading page treats that 45° as the triangle itself, so the degree scale stays unused for this mark. The same sloping edge is on a metric body. A 90 mm shelf end is the everyday case: hook the lip, draw the hypotenuse once, and the mitre is on the board with no pitch number. Degree figures are a separate row. A UK roof pitch in degrees belongs on the speed square rafter layout page. If the pivot arrived rounded, the lip rocks, so look at that corner before you cut.
Scribe a parallel line in the notches you actually have
A speed square scribe line sits parallel to the edge at one fixed offset. Seat the pencil in a notch, keep the lip on the edge, and pull the square along the board. The notch holds the pencil and the fence holds the distance. Lift the lip and the line drifts toward the middle of the board.
Notch spacing belongs to the model in your hand. Quarter-inch steps are the imperial spacing, including on the Swanson pair in this comparison. OX’s UK page for its 180 mm square cites 5 mm scribe notches. A metric Swanson in UK shops cites 0.5 cm notches, the same 5 mm step, and that shop model is a different tool from the imperial pair here. A quarter of an inch is about 6.4 mm, so inch-notch counts on a metric body miss the millimetre on the list. Holes from 3 mm to 15 mm, on Preciva and Difistik, are a drill gauge. A pencil in a 10 mm hole is sized for a bit. A smooth edge with no notches needs a marking gauge.
A short fence for the saw shoe
The same lip can be a speed square saw guide for a short cut. The shoe of a handheld circular saw can run against that raised edge when the cut is too short for a track. Support the offcut so it does not pinch the blade. The saw’s own guard stays down. The lip does not clamp itself, and a thin body is a poor fence on heavy timber because the shoe can push it off the line.
HSE publication WIS16 describes circular saw benches: the bench fence, the riving knife, and the push-stick. Those settings stay on that machine. At work, PUWER applies to the saw. If you cannot keep a bench saw guarded as WIS16 describes, stop and call someone who runs that saw. OX’s UK page calls the wide base a saw guide. That is the manufacturer’s fence claim, not an HSE approval. A crooked body then gives the shoe a crooked cut, which is why the flip test comes next. What to buy, including a thicker named base, is the speed square buying guide.
Flip the square before you trust it
A speed square is one piece. A centre punch sometimes trues a framing square, and it is the wrong repair here. To check a speed square is square, mark with the lip on one face of a straight edge, flip the fence to the other face at the same point, and mark again. Parallel lines, or one shared line, mean the body agrees with itself. A V means it has failed, and you replace it.
One shared Flintronic review says the large square is out by 0.7 mm. That is one buyer’s report, not a laboratory grade for the set, and it is a reason to test the square in your hand, including a new one. A failed flip on a structural cut means you stop and replace it. The Consumer Rights Act 2015 covers a new square that is not as described or not of satisfactory quality. None of these listings shows a BSI grade. A star rating is not a tolerance such as ±0.5°, and nothing here measured one. When the lines open, a pitch mark will not straighten the body.
Common questions
How do you mark 90° with a speed square?
Hook the lip flat against a straight edge and draw along the leg that meets the lip at a right angle. The line is square to the edge you hooked. A bowed edge produces a line that is square to the bow, so sight the edge first if the board has been stored on its face. The degree row is a separate mark, for an angle you choose rather than for this right angle.
How do you mark 45° without reading the degree scale?
Keep the lip on the edge and draw along the hypotenuse. Swanson’s reading page treats that 45° as the triangle itself, and a metric square has the same sloping edge. A mitre on a 90 mm shelf end needs no pitch number. Use the degree scale when the angle is neither 90° nor that built-in 45°.
How do you scribe a line parallel to the edge?
Seat the pencil in a notch, keep the lip on the edge, and pull along the board. Quarter-inch notches are the imperial spacing, including on the Swanson pair. OX cites 5 mm scribe notches on its metric 180 mm square, and a shop metric Swanson cites 0.5 cm, the same step. Holes from 3 mm to 15 mm are a drill gauge. Count the notches the square actually has.
Can the lip guide a circular saw?
The shoe of a handheld circular saw can run against the lip on a short cut when the stock is supported and the saw’s guard stays on. The lip does not clamp itself, and a thin body is a poor fence on heavy timber. HSE WIS16 covers bench saws, including the riving knife and the push-stick. Those settings stay on the bench saw. At work, PUWER applies to the saw.
How do you flip-test a speed square?
Mark a line with the lip on one side of a straight edge, flip the fence to the other side at the same point, and mark again. Parallel lines, or one shared line, mean the body agrees with itself. Lines that open into a V mean it has failed. You replace it. A centre punch suits a framing square you can true, and this triangle is a different tool.
What if a review says the square is 0.7 mm out?
The 0.7 mm figure is one shared Flintronic review, not a lab measurement of every square in this set. Run the flip test on the tool in your hand. If the lines open, stop before a structural cut and replace the square. A new square that is not as described or not of satisfactory quality falls under the Consumer Rights Act 2015. A star rating is the amazon.co.uk review line, not a calibration.
Where do roof cuts go?
Pitch in degrees, the common scale, and the hip-and-valley scale are on the rafter layout page. A UK drawing usually states the pitch as a degree. A rise-per-12 figure from a US video is a different system, and that page keeps the two apart. This page stops at the cross-cut, the scribe, the short fence, and the flip test.
Related pages
Sources
Public sources listed below. Prices and star lines are from the twelve amazon.co.uk listings captured on 19 October 2026.
- Swanson, Reading the Speed Square (retrieved 19 October 2026)
- OX Tools UK, 180 mm metric rafter square (retrieved 19 October 2026)
- DeWalt UK, DWHT25227-0 180 mm metric speed square (retrieved 19 October 2026)
- HSE, WIS16 Circular saw benches (retrieved 19 October 2026)
- BSI, BS 3322:1981 Carpenters' squares and bevels (retrieved 19 October 2026)
- BSI, BS 939:2007 Engineers' squares (retrieved 19 October 2026)
- legislation.gov.uk, Consumer Rights Act 2015 (retrieved 19 October 2026)
- Eden District Council, Pitched-roof timber sizes (retrieved 19 October 2026)