Needle and hook sizes across regions
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Needle and hook sizing is, if anything, worse than yarn weights. Three systems, two of which use bare numbers that mean different things, and one of which counts in the opposite direction.
Metric is the reliable one
Millimetres measure the actual diameter of the needle or hook. A 4mm needle is 4mm everywhere in the world. When a pattern gives a metric size, there is no ambiguity.
If you are buying tools and have a choice, metric-marked ones will save you trouble.
US sizes
Knitting needles count upward as they get thicker: US 0 is 2mm, US 8 is 5mm, US 15 is 10mm.
US crochet hooks use a letter-and-number scheme (B/1, E/4, H/8, K/10.5) which also ascends with thickness. Note that the letter and the number are two labels for the same size, not separate dimensions.
UK sizes
UK knitting needle sizes count downward. UK 14 is 2mm; UK 000 is 10mm. This catches people out constantly when working from older British patterns.
UK crochet hooks follow the same descending logic.
Common equivalents
| Metric | US knitting | UK knitting |
|---|---|---|
| 2.0mm | 0 | 14 |
| 2.75mm | 2 | 12 |
| 3.25mm | 3 | 10 |
| 3.75mm | 5 | 9 |
| 4.0mm | 6 | 8 |
| 4.5mm | 7 | 7 |
| 5.0mm | 8 | 6 |
| 5.5mm | 9 | 5 |
| 6.0mm | 10 | 4 |
| 8.0mm | 11 | 0 |
| 10.0mm | 15 | 000 |
Some sizes have no exact counterpart in another system and are rounded to the nearest available tool. Where that happens, the metric measurement is the one to trust.
How YarnHub handles this
Every tool size in the app is stored in metric internally and displayed in your chosen system. Set your region in your profile and pattern requirements, your equipment list, and catalogue recommendations all convert automatically.
Where a size has no exact equivalent in your system, YarnHub shows the nearest match and keeps the metric value visible so nothing is lost in translation.