~ FREE REFERENCE TOOL ~
Three sizing systems, two of which run backwards. Type any hook size in any system and get the rest, or read the full charts below — regular hooks, steel thread hooks, and knitting needles, with the yarn weight each size normally pairs with.
Type millimetres, a US letter or number, or a UK number.
⚠ That could mean more than one size, because the systems reuse numbers. All the matches are below.
UK numbers run backwards: the bigger the number, the smaller the hook. US letters run the normal way.
| mm | US | UK | Japan | Yarn weight |
|---|---|---|---|---|
| 2 | 0 | 14 | 2/0 | Super Fine |
| 2.25 | B-1 | 13 | 3/0 | Super Fine |
| 2.5 | — | — | — | Super Fine, Fine |
| 2.75 | C-2 | 12 | — | Super Fine, Fine |
| 3 | — | 11 | 5/0 | Fine |
| 3.25 | D-3 | 10 | — | Fine |
| 3.5 | E-4 | — | 6/0 | Fine, Light |
| 3.75 | F-5 | 9 | — | Light |
| 4 | G-6 | 8 | — | Light, Medium |
| 4.25 | G | — | — | Mediumℹ Boye labels its G as 4.25 mm; most brands call G 4.0 mm. |
| 4.5 | — | — | — | Medium |
| 4.75 | — | 7 | 7/0 | Medium |
| 5 | H-8 | 6 | 8/0 | Medium |
| 5.5 | I-9 | 5 | 9/0 | Medium, Bulky |
| 6 | J-10 | 4 | 10/0 | Bulky |
| 6.5 | K-10½ | 3 | — | Bulky |
| 7 | — | 2 | — | Bulky |
| 7.5 | — | 1 | — | Bulky |
| 8 | L-11 | 0 | — | Bulky, Super Bulky |
| 9 | M/N-13 | 00 | — | Super Bulky |
| 10 | N/P-15 | 000 | — | Super Bulky |
| 12 | O | — | — | Super Bulky, Jumbo |
| 15 | P/Q | — | — | Jumbo |
| 16 | Q | — | — | Jumbo |
| 19 | S | — | — | Jumbo |
| 25 | T/U/X | — | — | Jumbo |
| Weight | Also called | Hook | Needle |
|---|---|---|---|
| 0 · Lace | 2-ply, fingering, 10-count thread | 1.6–2.25 mm | 1.5–2.25 mm |
| 1 · Super Fine | sock, fingering, baby | 2.25–3.5 mm | 2.25–3.25 mm |
| 2 · Fine | sport, baby | 3.5–4.5 mm | 3.25–3.75 mm |
| 3 · Light | DK, light worsted | 4.5–5.5 mm | 3.75–4.5 mm |
| 4 · Medium | worsted, afghan, aran | 5.5–6.5 mm | 4.5–5.5 mm |
| 5 · Bulky | chunky, craft, rug | 6.5–9 mm | 5.5–8 mm |
| 6 · Super Bulky | super chunky, roving | 9–15 mm | 8–12.75 mm |
| 7 · Jumbo | jumbo, roving | 15–25 mm | 12.75–25 mm |
A hook has one physical measurement — the diameter of its shaft, in millimetres — and at least three names. The metric size is that diameter. The US size is a letter and number that ascends with it: B-1 is small, K-10½ is large. The UK/Canadian size is a number that descends: a UK 14 is a 2 mm hook and a UK 0 is an 8 mm one.
So a US H-8, a UK 6 and a 5 mm hook are the same object with three names, and two of those names imply opposite things about size. That is the whole reason this page exists, and it is why the Craft Yarn Council tells you to go by the millimetres.
Steel hooks invert again. Thread hooks for doilies and lace use their own numbers that run backwards relative to yarn hooks — a steel 12 is about 1 mm, a steel 00 is 3.5 mm. Anyone who learned "bigger number, bigger hook" from yarn hooks gets thread exactly wrong.
And steel sizing is genuinely not one system: an older European numbering circulates alongside the modern US one, which is why published charts contradict each other about what a "00" or a "12" is. The steel chart below carries only the modern US numbering rather than merging the two into something self-contradictory — and where a number is disputed, it says so.
Brands add their own drift on top. Boye labels its G hook 4.25 mm where most makers call G 4.0 mm. Several millimetre sizes have no US or UK name at all. If a pattern and your hook disagree, trust the millimetres and then trust your gauge swatch.
A US H-8 hook is 5.0 mm, which is a UK 6 and a Japanese 8/0. It is the size most worsted-weight (CYC 4) patterns assume.
The UK system numbers hooks by an old wire-gauge convention where a higher number meant thinner stock. US letters were assigned later and run the intuitive way. Neither is wrong; they just count in opposite directions, so a UK 14 (2 mm) and a US 14 mean completely different things.
Bulky (CYC 5) yarn normally takes a 6.5–9 mm hook — a US K-10½ up to an M/N-13. Super bulky (CYC 6) takes 9–15 mm. Those are starting points: check your gauge swatch, because tension varies more than hook size does.
No, and they run the other way. Steel hooks are for thread — roughly 0.75 mm to 3.5 mm — and their numbers descend as the hook grows, so a steel 12 is around 1 mm while a steel 00 is 3.5 mm. Two different steel numbering systems are also in circulation, so charts disagree. Go by millimetres.
Only through the millimetres. A 5 mm needle and a 5 mm hook are the same diameter, but the US numbering differs — a 5 mm needle is a US 8 while a 5 mm hook is a US H-8. The converter above will translate between the two for any diameter.
Measure the shaft — the straight part below the hook head, not the handle or the tapered throat — with digital calipers, and match the millimetres to the chart. Failing that, work a swatch and compare it against a hook you do know.
The gauge has to match; the hook is only how you get there. If your swatch comes out too big, go down a size, and if it comes out too tight, go up. That is normal and expected — pattern designers list the hook they used, not a rule.