SaveThree countries, three systems, and one of them runs backwards. Metric is the only one that describes something physical: a 5.0 mm hook is five millimetres across the shaft. US letters and UK numbers are conventions, and UK numbers get smaller as the hook gets bigger, which catches everyone exactly once.
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If a pattern gives only a letter or a number, find the millimetre size before you start. Letter sizing has drifted between manufacturers over the years and an old pattern may not mean what a modern hook says.
The full chart
| Metric | US | UK | Yarn weight | Typical use |
|---|---|---|---|---|
| 2.0 mm | – | 14 | Lace | Thread doilies, fine snowflakes |
| 2.25 mm | B-1 | 13 | Lace, fingering | Amigurumi in DK |
| 2.75 mm | C-2 | 12 | Fingering | Socks, fine amigurumi |
| 3.25 mm | D-3 | 10 | Fingering, sport | Small amigurumi |
| 3.5 mm | E-4 | 9 | DK | Amigurumi in worsted, tight fabric |
| 3.75 mm | F-5 | – | DK | DK garments |
| 4.0 mm | G-6 | 8 | DK | DK blankets and garments |
| 4.5 mm | 7 | 7 | DK, worsted | Dishcloths |
| 5.0 mm | H-8 | 6 | Worsted | The default. Most beginner patterns. |
| 5.5 mm | I-9 | 5 | Worsted | Blankets, better drape |
| 6.0 mm | J-10 | 4 | Worsted, aran | Loose blankets |
| 6.5 mm | K-10.5 | 3 | Bulky | Fast throws |
| 8.0 mm | L-11 | 0 | Bulky | Chunky scarves |
| 9.0 mm | M-13 | 00 | Super bulky | Arm-feel blankets |
| 10.0 mm | N-15 | 000 | Super bulky | Rugs, baskets |
| 12.0 mm | P | – | Jumbo | Held-double blankets |
UK numbering runs in reverse: a UK 14 is tiny and a UK 000 is enormous. If a pattern number seems to contradict the project, that is almost certainly why.
Which size to reach for
| You are making | Yarn | Hook | Why |
|---|---|---|---|
| Amigurumi | Worsted | 3.0 to 3.5 mm | Tight enough that stuffing cannot show |
| A blanket | Worsted | 5.5 mm | Half a size up from standard, for drape |
| A hat | Worsted | 5.0 mm | Standard, holds shape |
| A bag | Cotton | 4.0 mm | Down a size so it does not stretch |
| A dishcloth | Cotton | 4.5 mm | Dense enough to scrub |
| A garment | Whatever the pattern says | Match gauge | Fit depends on it |
When a pattern gives a hook size and a gauge and they disagree with your swatch, follow the gauge and change the hook. The hook size in a pattern is a starting suggestion; the gauge is the actual requirement.
Questions
What is the most common crochet hook size?
5.0 mm, marked H-8 in the US and 6 in the UK. It suits worsted yarn, which most free patterns are written for.
Why do UK hook numbers go backwards?
They are a legacy wire gauge system where a higher number means a thinner tool. It is counterintuitive and it is not going to change.
Does hook size change the finished size?
Considerably. Going up half a millimetre on a blanket can add ten centimetres across. That is what a gauge swatch tells you.
What hook for amigurumi?
About one and a half millimetres smaller than the yarn band says, so 3.0 to 3.5 mm for worsted. The fabric should feel too stiff.
Are US letters reliable?
Not entirely. Letter sizing has drifted between brands, so check the millimetre marking stamped on the hook itself.
What size should a beginner buy first?
One 5.0 mm hook with a soft handle. Buy other sizes as patterns actually ask for them.