Metric vs imperial speed square
Updated: 22 October 2026
You are at the bench with a millimetre tape, a drawing at 35°, and a video that says “6 in 12”. Metric vs imperial speed square is that one fork. Match the scale to the tape and the drawing: millimetres and degrees for ordinary UK work, and rise per 12 inches only when you are following that pitch on purpose. Prices are the amazon.co.uk figures from 19 October 2026, on the [speed square comparison](/hub/measuring-and-marking-tools/speed-square). Peter F. read those listings. No laboratory test. A qualifying purchase can earn us a commission at no extra cost to you.
You are at the bench with a millimetre tape, a drawing at 35°, and a video that says “6 in 12”. Metric vs imperial speed square is that one fork.
You are at the bench with a millimetre tape, a drawing at 35°, and a video that says “6 in 12”. Metric vs imperial speed square is that one fork. Match the scale to the tape and the drawing: millimetres and degrees for ordinary UK work, and rise per 12 inches only when you are following that pitch on purpose. Prices are the amazon.co.uk figures from 19 October 2026, on the speed square comparison. Peter F. read those listings. No laboratory test. A qualifying purchase can earn us a commission at no extra cost to you.
Our top picks
| Badge | Product | Scales | Size | Single or set | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  OX OX speed square set, 180 mm and 300 mm | Metric | 180 and 300 mm | Pair | 7.8 | about £29.79 | View offer |
| Alternative |  DeWalt DeWalt speed square, 18 cm | Metric | 180 mm | Single | 7.2 | about £16.25 | View offer |
| Alternative |  Swanson Swanson speed square kit, 7 inch and 12 inch | Imperial | 7 and 12 inch | Pair | 8.0 | about £21.84 | View offer |
On this page
The fork is the scale, not the brand
A UK cutting list is in millimetres, and a roof pitch on a UK drawing is usually a degree. A metric speed square prints those units, so it is the default when the tape already speaks them. You can transfer 38 mm or 1,200 mm onto a metric ruler without turning the mark into inches first. The brand on the heel does not do that job. Two squares from the same maker can be two languages, and the listing has to say which one you are holding.
Once the job is in millimetres, the next question is what a rise-per-12 number is asking you to read. On an imperial speed square, Swanson’s notes print pitch as rise per 12 inches of run. The number 6 on that common row means 6 inches of rise for 12 inches of run. It is a pitch sentence, not 6 mm and not a length on the tape. The degree row is what a UK drawing and a metric square can share. A title that says 7 inch is a trade size of about 180 mm. Several metric triangles here wear that nickname and still read in millimetres. Navoys clashes: the title says 7 inch, one bullet says 6 inch, and the bullets never name the unit. Until those lines agree, that row is not a fork you can check. Leg length stays on the speed square buying guide.
What the imperial numbers are
Swanson’s reading page separates four ideas, and mixing them is how a US video goes wrong on a UK board. Pitch is rise per 12 inches of run. The degree scale is its own row. The common scale is rafter length per foot of run. The hip-val scale is length per 17 inches of run. A pitch number is not the length of the piece. A 35° drawing can sit on the degree mark when the square prints degrees.
Swanson’s pitch-reference page states 5/12 as 22.62° and 6/12 as 26.57°. Those conversions belong to the inch-rise tool. This page leaves the pivot alone. DeWalt and OX are the metric rafter tools in this set. Their UK pages describe metric marks, and degrees, for common, hip, and valley cuts. They do not give a rise-per-10 formula. If the video says “6 in 12” and your square is metric, convert the pitch to a degree from a stated source, or pick up the imperial square. A printed 6 is not the same cut on both scales. The plumb cut is on the speed square rafter layout page.
Swanson is not one scale
In this set the imperial pair is Swanson SW1201K, ASIN B01CI4UBN6, the 7 inch and 12 inch kit at about £21.84, with 4.6 from 10,068 amazon.co.uk reviews on 19 October 2026. Graduations are in quarter inches, with a 44-page Blue Book and no layout bar. UK reviewers on that listing say they wanted metric. UK shops also sell a Swanson metric square at 250 mm, with metric or degree scales and 0.5 cm notches. Tool Britannia is one of those shops. That 250 mm square is outside this capture, so the name Swanson does not settle the fork. Its shop price, and the live price of the separate model S0101, stay off this page. S0101 is not this pair.
The metric rafter tools here are DeWalt and OX. DeWalt DWHT25227-0 is ASIN B09ZB5W993, the 18 cm listing at about £16.25. The UK page for that model also mentions 30 cm in an includes line. The amazon.co.uk listing is the 18 cm square alone. DWHT46031-0, a different string, is sold as a “7-in (180mm)” metric square. That inch phrase is a trade size, and that model is not DWHT25227-0. OX is ASIN B0H6JVW73D, the 180 mm and 300 mm pair at about £29.79, model OX-P712301-E. The OX UK 180 mm page uses OX-P634701, a different listing. Both are metric. Neither is the Swanson rise-per-12 pair.
Which side of the fork you are on
When the tape and the cutting list are in millimetres, and the pitch is a degree, buy a metric square. In this set that is the DeWalt 18 cm square at about £16.25, or the OX pair at about £29.79 when you also want the longer leg. When you are deliberately following a rise-per-12 video or an inch cutting list, buy Swanson SW1201K at about £21.84. That is the recommendation, with a condition: the imperial pair is the wrong default for a millimetre tape, even though 10,068 reviews is the largest count in the set. Stars count ratings. They do not convert the scale.
If a title says metric and the marks in your hand are in inches, the Consumer Rights Act 2015 covers goods that are not as described, and satisfactory quality. None of these twelve listings shows BS 939:2007 or BS 3322:1981. A seller line of ±0.2 mm is seller text, not a British Standard grade. A pitch mark on either scale does not size a rafter.
Common questions
If my tape is in millimetres, do I need a metric speed square?
For ordinary UK work, yes. A millimetre tape and a degree pitch match a metric ruler, so 38 mm or 1,200 mm transfers without an inch conversion. In this set that is the DeWalt 18 cm square at about £16.25 or the OX pair at about £29.79, from the 19 October 2026 amazon.co.uk capture. Rise per 12 inches is the other language. Use it when the job is written that way on purpose.
What does rise per 12 inches mean on a Swanson square?
The pitch number is inches of rise for 12 inches of run. Swanson’s reading page keeps that row separate from the degree scale, from the common length scale per foot of run, and from the hip-val length scale per 17 inches of run. The pitch number is an instruction about rise and run. A printed 6 is not 6 mm, and it is not the length of the rafter.
Can I follow a US framing video with a metric square?
You can follow the geometry once the pitch is in degrees, which is how a UK drawing usually states it. The video’s “6 in 12” is not the number 6 on a metric common scale. DeWalt and OX UK pages do not describe those squares as rise per 10. Convert to a degree from a stated source, or use the imperial Swanson pair at about £21.84. The pivot is on the rafter page.
Is a square sold as 7 inch the same as an imperial scale?
A 7 inch name is a trade size, often about 180 mm, and metric squares in this set wear it. The graduations are imperial only when the listing says so. Navoys clashes between 6 inch and 7 inch and never names the unit, so that row does not settle the fork. DeWalt’s DWHT46031-0 string pairs “7-in” with 180 mm on a metric square, and that string is not the 18 cm listing B09ZB5W993.
What if the title says metric and the marks are in inches?
The title and the scale are then in conflict. The Consumer Rights Act 2015 covers goods that are not as described and goods that are not of satisfactory quality. None of these twelve listings shows BS 939:2007 or BS 3322:1981. Read the scale before you trust the title, and read the marks again when the square arrives. A pitch figure on the wrong scale still does not size the rafter.
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)