Hook size by yarn weight, US letter to metric, UK conversion, and when to size up or down.
Crochet hooks are sized by diameter in millimetres — from 0.6mm (thread crochet) up to 25mm (jumbo). The most common general-purpose hooks are 5mm (US H/8) for worsted yarn and 4mm (US G/6) for DK yarn. Pattern designers list both metric and US-letter sizes; the metric measurement is more precise. To make amigurumi tighter, size down from the yarn's recommended hook by half a millimetre. For blankets that need drape, size up.
| Metric (mm) | US letter | UK | Best yarn weight | Common use |
|---|---|---|---|---|
| 2.25mm | B/1 | 13 | Lace / Crochet thread | Doilies, fine lace |
| 2.75mm | C/2 | 12 | Lace / Sock | Detailed lace |
| 3.25mm | D/3 | 10 | Sock / Fingering | Lightweight shawls, socks |
| 3.5mm | E/4 | 9 | Sport / DK | Baby items, fine amigurumi |
| 3.75mm | F/5 | 9 | DK / Light Worsted | Garments, blankets in DK |
| 4mm | G/6 | 8 | DK / Light Worsted | Standard DK projects |
| 4.5mm | 7 | 7 | DK / Worsted | Tight worsted, loose DK |
| 5mm | H/8 | 6 | Worsted | Most common — amigurumi, blankets |
| 5.5mm | I/9 | 5 | Worsted / Aran | Aran-weight projects |
| 6mm | J/10 | 4 | Aran / Bulky | Heavy garments, throws |
| 6.5mm | K/10½ | 3 | Bulky | Quick-make blankets |
| 8mm | L/11 | 0 | Super Bulky | Chunky throws, baskets |
| 9mm | M/13 | 00 | Super Bulky | Statement pieces |
| 10mm | N/15 | 000 | Jumbo / Roving | Oversized accents |
| 15mm+ | P/Q/S | — | Jumbo | Arm-crochet rugs |
Start with the size on the yarn's ball band — that's the gauge the manufacturer tested. Then check the pattern's recommended hook. For tighter fabric (amigurumi), size down 0.5mm. For drapier fabric (blankets, lightweight garments), size up 0.5mm.
The US letter system is inconsistent across hook brands. Metric millimetres are an exact measurement everyone agrees on.
What size crochet hook for beginners?
5mm (US H/8) with worsted-weight yarn is the standard beginner combination.
What hook size for amigurumi?
Most amigurumi patterns use 4mm or 4.5mm hook with worsted-weight yarn.
What is a US H crochet hook in mm?
US H = 5mm.
What is a 4mm crochet hook in US letter?
4mm = US G or G/6.
Should I size up or down on hook?
Size down for amigurumi (tighter fabric). Size up for blankets and wearables.
Do hook brands measure the same?
Metric millimetres are consistent. US letter sizes drift between brands.