Crochet hook sizes come in three systems: US letters and numbers, millimeters, and the old UK numbers. Only the millimeters are an actual measurement. The other two are labels that map onto it, and they do not map perfectly.
When a pattern and your hook disagree, trust the millimeters.
Conversion chart · Which hook for which yarn · Steel hooks · Why US letters vary · Choosing a size
Crochet hook size conversion chart
Standard hooks, smallest to largest. US sizes are shown as letter and number, since manufacturers print one or both. Rows marked with a dash have no US size.
| US | Metric | Old UK |
|---|---|---|
| – | 2 mm | 14 |
| B-1 | 2.25 mm | 13 |
| – | 2.5 mm | 12 |
| C-2 | 2.75 mm | 12 |
| – | 3 mm | 11 |
| D-3 | 3.25 mm | 10 |
| E-4 | 3.5 mm | 9 |
| F-5 | 3.75 mm | 9 |
| G-6 | 4 mm | 8 |
| 7 | 4.5 mm | 7 |
| H-8 | 5 mm | 6 |
| I-9 | 5.5 mm | 5 |
| J-10 | 6 mm | 4 |
| K-10.5 | 6.5 mm | 3 |
| – | 7 mm | 2 |
| L-11 | 8 mm | 0 |
| M/N-13 | 9 mm | 00 |
| N/P-15 | 10 mm | 000 |
| O-16 | 12 mm | – |
| P/Q | 15 mm | – |
| Q | 16 mm | – |
| S | 19 mm | – |
| T/U/X | 25 mm | – |
A 2.5 mm hook, for example, exists in metric and old UK sizing but was never given a US letter. If a pattern calls for one, buy it by the millimeters.
Which hook for which yarn
Yarn labels give a recommended hook size, and that is the place to start. When there is no label, these are the usual ranges by yarn weight.
| Yarn weight | Also called | Hook range |
|---|---|---|
| Lace (0) | cobweb, thread | 1.5 to 2.25 mm |
| Super fine (1) | fingering, sock, 4 ply | 2.25 to 3.5 mm |
| Fine (2) | sport, baby | 3.5 to 4.5 mm |
| Light (3) | DK, light worsted | 4.5 to 5.5 mm |
| Medium (4) | worsted, aran, afghan | 5.5 to 6.5 mm |
| Bulky (5) | chunky, craft, rug | 6.5 to 9 mm |
| Super bulky (6) | super chunky, roving | 9 to 15 mm |
| Jumbo (7) | jumbo, arm knitting | 15 mm and up |
These are the Craft Yarn Council ranges, and they are ranges on purpose. A worsted yarn worked at 5 mm makes a firm fabric for a basket. The same yarn at 6.5 mm makes a drapey fabric for a shawl. The hook is how you choose which.
Crochet generally uses a hook one or two sizes larger than the needle a knitter would use for the same yarn, because crochet fabric is denser and needs the extra room to drape.
Steel hooks for thread
Steel hooks for lace and thread crochet use a separate numbering, and it runs backwards: a bigger number is a smaller hook.
| US steel | Metric |
|---|---|
| 4 or 5 | 1.75 mm |
| 6 | 1.65 mm |
| 7 or 8 | 1.5 mm |
| 9 | 1.4 mm |
| 10 | 1.3 mm |
| 11 | 1.1 mm |
| 12 | 1 mm |
| 13 | 0.85 mm |
| 14 | 0.75 mm |
Steel sizes vary more by manufacturer than standard hooks do. A size 7 from one maker can be the same hook as a size 8 from another, and at this scale a tenth of a millimeter is a visible change in gauge. For thread work, buy hooks marked in millimeters if you can, so the number on the hook is the actual size and not one brand’s label for it.
Why two hooks marked G can be different sizes
US letters were assigned by manufacturers, not by a standard. Boye, Susan Bates, Clover, and others each chose their own mapping, and they still differ by a quarter millimeter in places. A G hook is 4 mm from most makers and 4.25 mm from some.
A quarter millimeter is enough to change your gauge. If you swatch with one brand and work the project with another, check the millimeters, not the letter.
The old UK numbers stopped being used for new hooks decades ago. They appear in vintage patterns and on hooks you inherit. Modern UK hooks are sold in millimeters, the same as everywhere else.
Choosing a hook size
Start with the size the pattern calls for, swatch, and measure. If you have more stitches per inch than the pattern, go up a size. Fewer, go down. Two people can use the same hook and yarn and get different fabric, which is why the size in the pattern is a starting point and the gauge is the target.
For a first hook, use a 5.5 mm (I-9) or 6 mm (J-10) with a worsted yarn. That is big enough to see what you are doing and small enough to be useful. There is a full beginner guide at How to Start Crocheting.
Everything Knotions publishes about crochet is in one place: The Knotions Crochet Authority. For the difference between US and UK stitch names, see US to UK Crochet Terms.
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