Crochet Hook Sizes Decoded: US Letters, UK Numbers, Metric mm and Japan

You’ve found the perfect free pattern online, you’ve got a bag of hooks, and then the instructions call for a “US H/8” — or worse, a “UK 6” — and your hooks are stamped only in millimeters. Or the reverse: a vintage British pattern lists a number that seems to have nothing to do with any hook you own. Crochet hook sizing is a genuine mess of overlapping national systems that grew up independently, and the labels don’t line up in any obvious way. The good news is that underneath all the letters and numbers there’s exactly one real measurement, and once you know it the confusion evaporates. The Crochet Hook Size Converter translates any size into all four systems instantly, but here’s what’s actually going on.

Millimeters are the only true measurement

Every crochet hook has a physical shaft diameter, and that diameter — expressed in millimeters — is the only thing that’s genuinely measured. A 4.0 mm hook is 4.0 mm across, full stop. The US letter “G/6,” the UK “8,” and the Japanese “7” are all just names different countries assigned to that same 4.0 mm shaft. So the moment you can find the mm value, you can find every other system’s name for it by looking it up in a table. The converter uses the Craft Yarn Council’s standard chart, which is the reference most modern pattern publishers follow.

This is why it’s worth buying hooks that are stamped with their millimeter size, or keeping a hook gauge (a little plate with graduated holes) in your project bag. If a hook has lost its label — and old aluminum hooks love to rub their markings off — you can push it through the holes of a gauge until you find the one it fits, read the mm, and you instantly know its name in every system. No memorization required.

Why UK numbers run backwards

Here’s the trap that catches everyone. In the US letter system, sizes go up as the hook gets bigger: B/1 is small (2.25 mm), G/6 is medium (4.0 mm), and by the time you reach J, K, L and beyond you’re into chunky-blanket territory. That’s intuitive — bigger letter, bigger hook. The metric system does the same thing: a bigger number is a bigger hook.

The UK number system does the exact opposite. UK numbers get smaller as the hook gets bigger. A UK 14 is a tiny 2.0 mm lace hook; a UK 8 is a middling 4.0 mm; and a UK 4 is a hefty 6.0 mm. The numbering descends toward zero and then rolls into “00” and “000” for the really big hooks. This is a leftover from an old wire-gauge convention, and it’s the single most common reason people grab completely the wrong hook when working from a British pattern. If you take one thing from this article, take that: on a UK pattern, a smaller number means a bigger hook. When in doubt, convert to mm and trust the millimeters.

Japanese sizing adds its own wrinkle — small hooks use their own number scale, but once you get past about 6 mm, Japanese hooks are often labeled directly in millimeters (e.g. “7 mm”), which is refreshingly honest. The US system also doubles up, pairing a letter with a number (G/6, H/8), and occasionally you’ll see the same letter mapped to two nearby sizes by different manufacturers, which is exactly why the mm value is the tiebreaker.

Matching hook size to yarn weight

Hook size and yarn weight are a partnership. Too small a hook for your yarn and the fabric comes out stiff and dense; too large and it’s loose, floppy and full of holes. The converter shows a recommended yarn-weight band for each hook, and the rough map runs like this:

• 1.5–3.5 mm — lace and fingering yarn, for fine shawls and thread crochet
• 3.5–4.5 mm — sport weight
• 4.5–5.5 mm — DK (double knitting) weight
• 5.5–6.5 mm — worsted / aran, the workhorse range for most garments and blankets
• 6.5–9 mm — bulky yarn, for fast, chunky projects
• 9–15 mm and up — super bulky and jumbo, for arm-warmer-thick fabric

These are starting points, not laws. Crochet, more than knitting, is often worked deliberately tighter or looser than the “recommended” hook to get a particular drape — amigurumi, for instance, is almost always made with a hook a size or two smaller than the yarn band suggests, specifically to create a dense fabric that won’t let the stuffing show through.

Why you should still make a gauge swatch

The uncomfortable truth is that the conversion chart gets you close, but it does not guarantee your project will come out the right size. Two hooks stamped “4.0 mm” from different manufacturers can vary by ±0.1 mm, and more importantly, every crocheter holds their yarn with different tension. That’s why patterns specify a gauge — a target number of stitches and rows over a set measurement — and why the pattern’s hook size is only a suggestion. If your swatch has too many stitches per inch, your tension is tight: go up a hook size. Too few, and you’re loose: go down. The pattern’s stated hook gets you into the right neighborhood; the swatch is what actually lands you on the doorstep. Skipping it on a fitted garment is how a sweater ends up two sizes off.

Convert once, then crochet with confidence

Crochet hook sizing looks chaotic only because four countries never agreed on a naming scheme — but they all describe the same physical shaft, and millimeters are the Rosetta Stone. Punch any size you have (or any size a pattern calls for) into the Crochet Hook Size Converter and it returns the mm, US, UK and Japanese equivalents plus the yarn weight that pairs with it, so you can shop, substitute, and follow patterns from anywhere without second-guessing. Then make a quick swatch to confirm your gauge, and adjust a size up or down if your tension needs it. If your project mixes crafts, the Knitting Gauge Converter handles the stitch-count math on the knitting side, and the Fabric Yardage Calculator covers you when a make calls for backing or lining fabric.