Trust the millimetres
A US crochet hook label often pairs a letter with a number, such as G-6 or H-8; some sizes have only one. The Craft Yarn Council notes that letter and number sizing varies from company to company, so use the millimeter size printed on the package when comparing hooks.
| Millimetres | US label |
|---|---|
| 4 mm | G-6 |
| 5 mm | H-8 |
| 6 mm | J-10 |
The full list is on the crochet hook sizes chart.
Steel hooks run the other way
Steel hooks, used for thread and lace-weight yarn, have their own numbers. For steel hooks, the higher the number, the smaller the hook, the reverse of regular hooks.