Three labelling systems, one hook. Convert between millimetres, US letters and UK numbers, and see which yarn weight each hook is for.
| Metric (mm) | US size | UK / Canada | Yarn weight |
|---|---|---|---|
| 2.0 mm | — | 14 | Lace / thread |
| 2.25 mm | B/1 | 13 | #0–1 Lace, fingering |
| 2.5 mm | — | 12 | #1 Fingering |
| 2.75 mm | C/2 | 11 | #1 Fingering |
| 3.0 mm | — | 11 | #1–2 Fingering, sport |
| 3.25 mm | D/3 | 10 | #2 Sport |
| 3.5 mm | E/4 | 9 | #2–3 Sport, DK |
| 3.75 mm | F/5 | 9 | #3 DK |
| 4.0 mm | G/6 | 8 | #3 DK |
| 4.5 mm | 7 | 7 | #3–4 DK, worsted |
| 5.0 mm | H/8 | 6 | #4 Worsted / aran |
| 5.5 mm | I/9 | 5 | #4 Worsted / aran |
| 6.0 mm | J/10 | 4 | #4–5 Aran, bulky |
| 6.5 mm | K/10½ | 3 | #5 Bulky |
| 7.0 mm | — | 2 | #5 Bulky |
| 8.0 mm | L/11 | 0 | #5–6 Bulky |
| 9.0 mm | M/N-13 | 00 | #6 Super bulky |
| 10.0 mm | N/P-15 | 000 | #6 Super bulky |
| 12.0 mm | O-16 | — | #6–7 Jumbo |
| 15.0 mm | P/Q | — | #7 Jumbo |
| 16.0 mm | Q | — | #7 Jumbo |
| 19.0 mm | S | — | #7 Jumbo |
Steel thread hooks use a separate scale (US 14 = 0.75 mm … US 00 = 3.5 mm) and get smaller as the number rises.
Start from the yarn label, then swatch. If your 4 in swatch has fewer stitches than the pattern's gauge, your hook is too big — go down a size; more stitches, go up. For amigurumi, deliberately use a hook one or two sizes smaller than the label so stuffing can't show through. For drapey shawls and garments, go one size up.
A US H/8 hook is 5.0 mm (UK size 6). It's the most common beginner hook because it suits #4 worsted/aran yarn.
The old UK/Canadian system counts down as hooks get bigger (UK 6 = 5 mm, UK 3 = 6.5 mm), a legacy of wire gauges. Modern hooks print the mm size — always trust the millimetres.
Start with the size printed on the yarn label, swatch, and adjust: a bigger hook for more drape, a smaller one for firm fabric or amigurumi (usually one size below the label).
A 5 mm (US H/8, UK 6) with #4 worsted yarn — large enough to see your stitches, small enough to control.
Related: Chain calculator (uses your gauge) · Yarn calculator · US ↔ UK terms
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