Knitting needle sizes
21 needle sizes in millimetres, US and UK numbers, with the yarn each suits.
The two scales run in opposite directions
This is the thing to know before reading any pattern. American numbers get bigger as the needle gets thicker. British numbers get smaller. So:
| Written | Means, in America | Means, in Britain |
|---|---|---|
| size 8 | 5 mm | 4 mm |
A whole millimetre on a 4 mm needle is about 25% of the diameter, which over a 100-stitch row is the difference between a garment that fits and one that does not. A pattern that says "size 8" without saying whose is not enough information.
They meet at exactly one size
Size 7 is 4.5 mm in both systems — the single point where the two scales cross, and the only number you can read from either country without checking. Everywhere else they diverge, and the gap widens in both directions.
The full chart
| Millimetres | US | UK | Yarn weight |
|---|---|---|---|
| 2 | 0 | 14 | Lace, 2-ply |
| 2.25 | 1 | 13 | Lace to fingering |
| 2.75 | 2 | 12 | Fingering, 4-ply |
| 3 | 2/3 | 11 | Fingering |
| 3.25 | 3 | 10 | Fingering to sport |
| 3.5 | 4 | — | Sport |
| 3.75 | 5 | 9 | Sport to DK |
| 4 | 6 | 8 | DK |
| 4.5 | 7 | 7 | DK to worsted |
| 5 | 8 | 6 | Worsted, aran |
| 5.5 | 9 | 5 | Aran |
| 6 | 10 | 4 | Aran to chunky |
| 6.5 | 10.5 | 3 | Chunky |
| 7 | — | 2 | Chunky |
| 8 | 11 | 0 | Chunky to bulky |
| 9 | 13 | 00 | Bulky |
| 10 | 15 | 000 | Super bulky |
| 12 | 17 | — | Super bulky |
| 15 | 19 | — | Jumbo |
| 19 | 35 | — | Jumbo |
| 25 | 50 | — | Jumbo |
The bold row is the one size where the two numbers agree. A dash means that scale has no number for that diameter.
Why the British scale counts backwards
It is a wire gauge. Needles were drawn from wire, and a gauge number counts the drawing operations it took — more passes, thinner wire, higher number. AWG counts the same way and for the same reason, which is why 12 gauge wire is thicker than 20.
The scale then runs out of numbers. Below 1 there is nothing left to count down to, so it starts adding zeros: 0, then 00, then 000, each thicker than the last — the same escape the wire gauges use when they reach 1/0, 2/0 and beyond. It is the clearest sign that these numbers were never a measurement.
The American scale has no such history. It simply numbers the needles from finest to thickest, and the two systems were never designed to be read against each other.
Buy by millimetre
The millimetre figure is the actual diameter and the only one that survives crossing a border, a brand or a decade. Most needles are marked with it, most modern patterns give it alongside the numbers, and where a pattern gives only a number the sensible move is to find out which country wrote it before casting on.
And whichever size a pattern names, the gauge swatch decides. Two knitters with the same needle and the same yarn routinely differ by half a needle size in tension, which is why every pattern that matters specifies stitches per 10 cm rather than trusting the number on the needle.
Wire gauge — the same backwards-counting logic. Unit converters