Crochet hooks are sized in millimeters, with US letter or number labels. This chart lines them up with the yarn weights they suit.
| Metric (mm) | US size | Typical yarn |
|---|---|---|
| 2.25 | B-1 | Fingering |
| 2.75 | C-2 | Sport |
| 3.25 | D-3 | Sport / DK |
| 3.5 | E-4 | DK |
| 4.0 | G-6 | Worsted |
| 5.0 | H-8 | Worsted |
| 5.5 | I-9 | Aran |
| 6.0 | J-10 | Aran / bulky |
| 6.5 | K-10.5 | Bulky |
| 8.0 | L-11 | Bulky |
| 9.0 | M/N-13 | Super bulky |
| 10.0 | N/P-15 | Super bulky |
Go by millimeters. US letter labels vary by brand, so the millimeter size is the reliable one. Match it to your pattern and gauge.
See the Crochet Hook Comparison and the Yarn Weight Chart.