Crochet Hook Size Chart & Conversion Guide

Crochet hook sizing is a mess of three competing systems, and the world has never quite agreed to pick one. The US labels hooks with letters, or numbers, or both. The UK and Canada use their own numbers, which run backwards. And the rest of the world sensibly just measures the thing in millimetres.

The millimetre is the only number you should actually trust. That is not our opinion — it is the Craft Yarn Council's own advice: "Because letter and number sizing vary from company to company, rely on the package millimeter (mm) sizing, which is an accurate measurement." The mm is a physical measurement of the hook's shaft, so it means the same thing no matter who made it. Letters are, effectively, marketing.

This page gives you the full conversion in every direction, and — more usefully than most charts — it is honest about the rows where the sources genuinely disagree, instead of quietly picking one and hoping you don't notice.

These are the standard aluminium, plastic and ergonomic hooks — the ones you use for anything from sock yarn up to jumbo blanket yarn. The millimetre and US columns are transcribed from the Craft Yarn Council's published chart, including the in-between sizes most charts drop.

The sizes brands actually fight about

Most of this chart is uncontroversial. A few rows are not, and these are the ones that quietly ruin gauge:

• G — 4.0mm or 4.25mm? Both. The CYC chart lists 4.0mm as "G-6" and 4.25mm as a letter-only "G". Boye is the brand you'll most often find the 4.25mm on.
• M vs N at 9mm and 10mm. CYC prints these as "M/N-13" and "N/P-15" — the slashes are not a typo, they are the CYC declining to referee. The letter N genuinely lands on both 9mm and 10mm depending on the brand.
• Q — 15mm, 15.75mm, or 16mm? The CYC chart lists a Q at all three. This is the single least reliable letter in crochet. Buy by millimetre here, always.
• Letter-only sizes. 3.125mm "D", 4.25mm "G", 5.25mm "I" and 5.75mm "J" all exist as letters with no number attached, and they sit right next to the numbered versions (D-3, G-6, I-9, J-10). If a vintage pattern calls for a bare letter, this is why.

The practical rule: when a pattern gives you a letter and a millimetre and they disagree with your hook, follow the millimetre. When a pattern gives you only a letter, treat it as a starting suggestion and let your gauge swatch make the final call.

Steel hooks are a separate universe. They are for crochet thread — the fine cotton used for doilies, snowflakes, lace edgings and bookmarks — and they have their own numbering that behaves nothing like letter hooks.

⚠️ Steel hooks run backwards. A size 14 steel hook (0.75mm) is the smallest hook you can buy. A size 00 (3.5mm) is the biggest. The number goes up as the hook gets tinier — the exact opposite of how millimetres work. In the CYC's own words: "The higher the number, the smaller the hook, which is the reverse of regular hook sizing."

Every yarn label suggests a hook, and that suggestion is a good default — it is the size that produces a fabric with normal drape for that yarn. Here is the Craft Yarn Council's recommended range for each of the eight standard yarn weights. For much more on the weight system itself, see our yarn weight chart and substitution guide.

Knowing the parts makes patterns — and hook shopping — make a lot more sense.

Inline vs tapered: Susan Bates and Boye

The two classic American hook brands are shaped differently, and crocheters have strong opinions about it. Susan Bates hooks are inline — the head is the same width as the shaft, with a squared-off throat and a deep groove that grips the yarn firmly. That tends to produce very even stitches, so they're favoured for accuracy and tight work. Boye hooks are tapered — the head flares out from the shaft with a shallower throat, so yarn slides off more easily and you can move faster.

Neither is better. Which one feels right depends on your grip, your tension and your yarn — so if crochet has ever felt like a fight, it is genuinely worth trying the other shape before you blame yourself.

Why hook size matters more than it looks like it should

Half a millimetre sounds like nothing. It is not. Your hook size controls the size of every single loop, and those loops multiply. On a beanie, one size off can mean a hat that won't stay on — or one that leaves a band mark on your forehead. On a cardigan, it can mean two full sizes of difference in the finished garment.

This is also why hook size is the lever you pull to fix gauge. If your swatch has too many stitches per inch, your fabric is too tight — go up a hook size. Too few stitches per inch means it's too loose — go down. Never change your yarn to fix gauge when changing your hook will do it.

Projects where being off by one size barely matters: scarves, blankets, baskets and letters — anything where the finished dimensions are flexible. Projects where it matters a lot: hats, garments, socks, and anything that has to fit a body.

More crochet references

• Yarn Weight Chart: 0–7 Explained (+ Substitution Guide) — which yarn to pair with which hook.
• Crochet Abbreviations & Terms: US/UK Translation Chart — decode any pattern.
• Free crochet doily patterns — where those steel hooks finally earn their keep.
• Beginner crochet scarf patterns — the best place to test a new hook.
• The Crochet Christmas Planner — includes a printable hook & notions inventory.

Sources. Hook and steel-hook millimetre/US values are transcribed from the Craft Yarn Council's Hook & Needle Size Charts and Steel Crochet Hook & Thread Sizes; hook-by-yarn-weight ranges from the Standard Yarn Weight System. UK/Canada numbers are not published by the CYC and were cross-checked against secondary hook guides. Where sources conflict, we've said so in the table rather than picked a side.