Hook size is measured in millimeters, based on the diameter of the shaft. That millimeter number is the only one that stays consistent across brands and countries — the letters and numbers stamped next to it vary, so treat them as a rough label, not the real measurement.
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US, UK, and metric sizes side by side
| Metric (mm) | US | Old UK | Typical yarn weight |
|---|---|---|---|
| 2.25 mm | B-1 | 13 | Lace / fingering (thread work) |
| 2.75 mm | C-2 | 12 | Fingering |
| 3.25 mm | D-3 | 10 | Sport |
| 3.5 mm | E-4 | 9 | Sport / DK |
| 3.75 mm | F-5 | 9 | DK |
| 4 mm | G-6 | 8 | DK / worsted |
| 4.5 mm | 7 | 7 | Worsted |
| 5 mm | H-8 | 6 | Worsted (the most common beginner size) |
| 5.5 mm | I-9 | 5 | Worsted / heavy worsted |
| 6 mm | J-10 | 4 | Bulky |
| 6.5 mm | K-10½ | 3 | Bulky |
| 8 mm | L-11 | 0 | Super bulky |
| 9 mm | M/N-13 | 00 | Super bulky |
| 10 mm | N/P-15 | 000 | Jumbo |
How to pick a hook when you don’t have a pattern
Check the yarn label first. Most yarn bands list a recommended hook range on the back — that’s your starting point, not a rule. Work a small swatch, roughly 4 inches square, and compare it to the pattern’s stated gauge (usually given as stitches and rows per 4 inches).
- If your swatch has more stitches per 4 inches than the pattern calls for, your stitches are too tight. Go up a hook size.
- If your swatch has fewer stitches, your stitches are too loose. Go down a hook size.
Gauge matters most for anything that needs to fit a body or a specific space — garments, hats, blankets meant for a certain bed size. For amigurumi or small motifs, gauge is more about matching the tightness the designer used (so stuffing doesn’t show through gaps) than hitting an exact measurement.
Why hook material changes how a size “feels”
Two hooks marked the same millimeter size can behave differently depending on material:
- Aluminum is smooth and lets yarn glide fast — good default, can feel slippery with silky or plant-fiber yarns.
- Wood or bamboo has a little more grip, which helps control slippery yarns but slows down the motion slightly.
- Inline heads (the throat is the same width as the shaft) tend to give more consistent stitch height — Susan Bates Silvalume hooks are a common inline example. Tapered heads narrow gradually and are easier to slide into tight stitches, like the top of an amigurumi round — Clover Amour is a common tapered example.
If your gauge swatch comes out consistently off from every pattern you try, the mismatch is sometimes the hook style, not the size — worth testing an inline hook against a tapered one in the same millimeter size before assuming your tension is the problem.
Steel hooks use a separate, reversed numbering system
The chart above covers standard hooks for yarn, but steel hooks made for fine thread work use their own numbering that runs the opposite direction: a higher steel hook number means a smaller, finer hook, the reverse of how regular hook letters and numbers work. A US steel size 12 (about 0.75 mm) is much finer than a steel size 0 (about 3.25 mm), so if you’re moving from yarn crochet into doily, thread, or fine lace work, don’t assume the numbering logic carries over, check the actual millimeter size on any steel hook rather than relying on the number alone.
Thumb rests and grip shape affect long sessions more than size does
Beyond material, a hook’s handle shape matters for comfort during long projects, a flat thumb rest gives you a visual and physical reference point for keeping the hook oriented consistently, useful for beginners still building muscle memory for hook rotation. An ergonomic, molded handle distributes grip pressure across more of your hand rather than concentrating it at your fingertips, worth the investment if you’re working on anything large enough to take multiple sessions, hand fatigue from a poor grip shape often gets blamed on hook size or yarn weight when the handle design is the actual culprit.