Crochet hooks use a letter-and-number US system for regular yarn hooks and a separate, reversed numeric scale for steel hooks used in thread crochet. The two scales aren't related, and mixing them up is a common source of confusion when a pattern just says "size 7" without specifying which system.
| US | Metric (mm) |
|---|---|
| B-1 | 2.25mm |
| C-2 | 2.75mm |
| D-3 | 3.25mm |
| E-4 | 3.5mm |
| F-5 | 3.75mm |
| G-6 | 4.0mm |
| 7 | 4.5mm |
| H-8 | 5.0mm |
| I-9 | 5.5mm |
| J-10 | 6.0mm |
| K-10.5 | 6.5mm |
| L-11 | 8.0mm |
| M/N-13 | 9.0mm |
| N/P-15 | 10.0mm |
| P/Q | 15.0mm |
| Q | 15.75mm |
| S | 19.0mm |
UK crochet hooks are sold almost entirely by metric size today, and the historical UK-specific hook numbering varied between manufacturers enough that no single conversion table is reliable. If a UK pattern lists a hook size, it's most likely already in millimeters, or it's referencing the same US letter system that's become the de facto international standard.
| US | Metric (mm) |
|---|---|
| 00 | 3.5mm |
| 0 | 3.25mm |
| 1 | 2.75mm |
| 2 | 2.25mm |
| 3 | 2.1mm |
| 4 | 2.0mm |
| 5 | 1.9mm |
| 6 | 1.8mm |
| 7 | 1.65mm |
| 8 | 1.5mm |
| 9 | 1.4mm |
| 10 | 1.3mm |
| 11 | 1.1mm |
| 12 | 1.0mm |
| 13 | 0.85mm |
| 14 | 0.75mm |
Notice the steel scale runs the opposite direction from the regular scale: a steel size 00 is a comparatively large 3.5mm hook, while a steel size 14 is a fine 0.75mm hook for the thinnest crochet thread. If a pattern's hook size seems oddly large or small for the project, checking which scale it's using usually clears it up.