Tools · Knitting & Crochet
Needle and hook sizes, converted honestly
Three systems name the same tools, and they do not agree with each other. Millimetres measure the tool itself. US numbers climb as tools get bigger. Old UK numbers fall. There is no formula that survives the crossing, so this page is a lookup, not a calculation. Pick the size your pattern names, in whichever system it names it, and read off the tool you actually own.
5.0 mm
- US
- 8
- Old UK
- 6
- Usually works
- Worsted and aran weight
the sweater size. you probably own three of these
Why mm wins every argument
A needle or hook size is a diameter, and millimetres state it directly. The other two systems are labels pinned to that diameter at different times by different countries, and the pins do not line up. US needle sizes skip numbers entirely: there is no US 12 and no US 14 needle, because the scale jumps from 11 to 13 to 15. The US hook at 4.5 mm is sold as a plain 7, with no letter at all. Old UK numbers run backwards, so a UK 3 is a chunky 6.5 mm tool while a UK 13 is a fine 2.25 mm one. Anyone who tries to convert by adding or subtracting will land on a tool that does not exist, or worse, one that does.
So the rule on this page, and in the Knotorious app, is simple: store the size in millimetres, treat US and UK as display labels, and when a pattern gives you two systems that disagree, believe the millimetres and check your gauge. Gauge is the final judge anyway. A half millimetre of brand variation disappears into a swatch. A whole system mixup does not.
if it is not stamped, measure it. the drawer lies
Have a tool with no size stamped on it?
- Needles: push the shaft through a needle gauge, the card with graded holes sold for exactly this. The smallest hole it fits snugly is its size. No gauge? Lay it on a ruler and compare against a stamped needle side by side; half a millimetre is visible when two shafts touch.
- Hooks: measure the shaft, the straight section your loops sit on, not the flattened thumb rest and not the head. A caliper reads it exactly; the needle gauge trick works too, using the shaft just behind the throat.
- Write the mm on it. A dot of nail varnish code or a labelled pouch beats re-measuring the same mystery hook every spring. Your future self, mid-project, thanks you.
The full needle table
| mm | US | Old UK | Usually works |
|---|---|---|---|
| 2.0 mm | 0 | 14 | Lace to fingering, socks and fine lace |
| 2.25 mm | 1 | 13 | Socks, fingering weight, fine lace |
| 2.5 mm | 1.5 | no old UK number | Socks and fingering weight |
| 2.75 mm | 2 | 12 | Fingering and sport weight |
| 3.0 mm | 2.5 | 11 | Sport weight and light garments |
| 3.25 mm | 3 | 10 | Sport and DK weight |
| 3.5 mm | 4 | no old UK number | DK weight |
| 3.75 mm | 5 | 9 | DK and light worsted |
| 4.0 mm | 6 | 8 | Worsted weight, the all rounder |
| 4.5 mm | 7 | 7 | Worsted and aran weight |
| 5.0 mm | 8 | 6 | Worsted and aran weight |
| 5.5 mm | 9 | 5 | Aran and bulky weight |
| 6.0 mm | 10 | 4 | Bulky weight |
| 6.5 mm | 10.5 | 3 | Bulky weight |
| 7.0 mm | no standard US size | 2 | Bulky weight, fast blankets |
| 8.0 mm | 11 | 0 | Bulky and super bulky |
| 9.0 mm | 13 | 00 | Super bulky weight |
| 10.0 mm | 15 | 000 | Super bulky and jumbo |
| 12.75 mm | 17 | no old UK number | Jumbo weight, arm knitting territory |
| 15.0 mm | 19 | no old UK number | Jumbo weight |
The full hook table
| mm | US | Old UK | Usually works |
|---|---|---|---|
| 2.25 mm | B-1 | 13 | Lace and fingering weight |
| 2.75 mm | C-2 | 12 | Fingering and sport weight |
| 3.25 mm | D-3 | 10 | Sport weight |
| 3.5 mm | E-4 | 9 | Sport and DK weight |
| 3.75 mm | F-5 | no old UK number | DK weight |
| 4.0 mm | G-6 | 8 | DK and worsted weight |
| 4.5 mm | 7 | 7 | Worsted weight; the US size with no letter |
| 5.0 mm | H-8 | 6 | Worsted weight, the amigurumi starter size |
| 5.5 mm | I-9 | 5 | Worsted and bulky weight |
| 6.0 mm | J-10 | 4 | Bulky weight |
| 6.5 mm | K-10.5 | 3 | Bulky weight |
| 8.0 mm | L-11 | 0 | Bulky and super bulky |
| 9.0 mm | M/N-13 | 00 | Super bulky weight |
| 10.0 mm | N/P-15 | 000 | Super bulky and jumbo |
| 15.0 mm | P/Q | no old UK number | Jumbo weight |
Two honest warnings about these tables. First, old UK numbers are approximate: they come from a wire gauge era, brands rounded differently, and a vintage UK 9 hook can sit a fraction off a modern 3.5 mm. Second, above 9 mm the US letters themselves wobble between brands, which is why 9.0 mm appears as M/N-13 and 10.0 mm as N/P-15. At the chunky end, millimetres are not just the best answer, they are the only one.
Steel hooks for thread crochet use their own numbering, and it runs in reverse: the higher the number, the smaller the hook. A steel 10 is far finer than a steel 1. They share nothing with the tables above except the word hook. If your pattern is for thread or lace weight cotton, check the millimetres twice and ignore the number until you have.
Then check the number that actually decides fit
Converting the tool gets you to the starting line. Whether the fabric comes out the right size is a gauge question, and gauge moves with your hands, your yarn and your mood. Work the swatch, measure the middle, and if the count is off, change one tool size and swatch again. That guide walks the whole fix, including the one formula worth memorising.