Hook sizes are labelled three different ways and none of them agree. A US pattern says H/8, a British one says 6, and the hook in your hand says 5 mm — all three are the same hook. This chart lines up all three systems, tells you which yarn weight each hook is meant for, and converts any size you type. Tap a row or a yarn card and the matching sizes light up on both.
Looking for the interactive converter with AU and Japanese sizes and ply numbers? That lives on the hook & yarn converter. This page is the reference chart — the one to print and tape inside the yarn cupboard.
Hook Size Conversion Chart
Click any row to see its matching yarn weight. Highlighted rows match the selected yarn weight.
| US Size | Metric (mm) | Old UK / Canadian (approx.) | Pairs With |
|---|
Yarn Weight Guide
Click a card to highlight its recommended hook sizes above.
Quick Converter
Type any hook size format and see all equivalents instantly.
Results will appear here as you type
US vs UK Crochet Terms
The same stitch can have different names depending on the pattern’s origin.
| US Term | UK Term | |
|---|---|---|
| slip stitch (sl st) | ↔ | slip stitch (sl st) |
| chain (ch) | ↔ | chain (ch) |
| single crochet (sc) | ↔ | double crochet (dc) |
| half double crochet (hdc) | ↔ | half treble (htr) |
| double crochet (dc) | ↔ | treble (tr) |
| treble crochet (tr) | ↔ | double treble (dtr) |
How the three hook systems line up
Millimeters are the only measurement that is actually a measurement. A 5 mm hook has a shaft 5 mm across, everywhere in the world, in every brand. The US letter-and-number labels (B/1 through S) and the old UK/Canadian numbers are naming conventions layered on top of that, and they were never standardised against each other.
The US system runs the way you would expect: letters get later as the hook gets bigger — B/1 is 2.25 mm, H/8 is 5 mm, K/10.5 is 6.5 mm. The old UK system runs backwards, because it borrowed its numbers from wire gauge: the number falls as the hook grows. UK 13 is 2.25 mm; UK 3 is 6.5 mm. Above about 8 mm the UK system simply stops and hooks are sold in millimeters only, which is why the largest rows in the chart show a dash.
The fourth column is the part people actually need. Each yarn weight has a hook range printed on its ball band, and the chart maps every hook into the weights it falls inside. The rule the table uses is straightforward: a hook pairs with a yarn weight when its millimeter size falls inside that weight’s published hook range — for example worsted (#4 medium) covers 5.5–6.5 mm, so I/9, J/10 and K/10.5 all sit in it.
A worked example
Say you have inherited a pattern that calls for a UK size 6 hook and a hook tin full of unlabelled aluminium.
- Find UK 6 in the third column. Its metric size is 5 mm.
- Read across: the US label is H/8. That is the number stamped on most American hooks.
- The pairing column says 5 mm sits inside #3 Light (DK) only — the published DK hook range is 4.5–5.5 mm, and worsted does not start until 5.5 mm. So the pattern is written for DK, not worsted.
- Check the gauge card: DK runs roughly 12–17 single crochet in 4 inches. If your swatch comes out at 19 sc in 4 inches you are crocheting tight — move up to I/9 (5.5 mm), which sits in both DK and worsted, and swatch again.
Two hooks marked the same letter by different brands can still be a quarter-millimeter apart. If a project is gauge-critical, trust the swatch over the label every single time.
Questions people ask
What is a US F/5 crochet hook in UK sizes?
A US F/5 hook is 3.75 mm, which maps to approximately old UK size 9. UK conversions are approximate — millimeters are the reliable measurement, so check the mm size on your hook.
What is the difference between US and UK crochet terms?
UK terms are one step “up”: a US single crochet (sc) is a UK double crochet (dc), a US double crochet is a UK treble (tr), and a US half double crochet is a UK half treble (htr). Always check a pattern’s origin before starting.
What size crochet hook for worsted weight yarn?
Worsted (#4 medium) yarn typically pairs with hooks I/9 to K/10.5 (5.5–6.5 mm). Check your yarn label and swatch for gauge-critical projects.
How do I convert a crochet hook size?
Type any format into the quick converter — “5mm”, “H/8”, or “UK 6” — and it shows the metric, US, and old UK/Canadian equivalents plus the yarn weights that hook pairs with. Millimeters are always the controlling size.
Why do UK crochet hook numbers run backwards?
The old UK and Canadian system numbers the hooks by wire gauge, and a wire gauge number gets smaller as the wire gets thicker. So UK 13 is a 2.25 mm hook and UK 3 is a 6.5 mm hook — the number falls as the hook grows. US letters run the other way: B is small, S is enormous. That is why millimeters are the only size worth trusting when you buy.