Knitting Needle & Crochet Hook Size Converter (mm, US, UK)
Convert knitting needle sizes between metric millimetres, US numbers and old UK/Canadian sizes, and crochet hooks between mm and US letters — with the yarn weight each size suits.
How it works
Look-up in the Craft Yarn Council standard chart: metric diameter (mm) ↔ US needle number ↔ old UK/Canadian number; crochet hooks mm ↔ US letter-number · millimetre entries snap to the nearest standard size
The metric size is the needle or hook diameter in millimetres and is the only system that means the same everywhere. US needle numbers go up as needles get thicker (US 8 = 5 mm), while the old UK and Canadian numbers run the other way (UK 6 = 5 mm). Crochet hooks use a letter plus a number in the US (H-8 = 5 mm) and millimetres elsewhere. Some metric sizes have no US or UK equivalent, shown as a dash. Brands differ by a fraction of a millimetre, so trust the mm marking and your gauge swatch.
Worked example
A US 8 knitting needle is 5 mm, which is a UK/Canadian size 6, and it suits medium (4) yarn — worsted or aran weight.
Sources: Craft Yarn Council — Hooks and needles · Craft Yarn Council — Standard yarn weight system
Frequently asked questions
What is a US size 8 needle in mm?
5 mm. US 6 is 4 mm, US 7 is 4.5 mm, US 9 is 5.5 mm and US 10 is 6 mm.
Why do UK needle numbers go backwards?
Old UK and Canadian sizes came from the standard wire gauge, where a bigger number means thinner wire. So UK 14 is a thin 2 mm needle and UK 000 is a thick 10 mm one.
What size is an H crochet hook?
An H-8 hook is 5 mm. G-6 is 4 mm, I-9 is 5.5 mm, J-10 is 6 mm and K-10½ is 6.5 mm.
Can I use a needle that is 0.25 mm different?
Yes, as long as you knit a gauge swatch. A quarter millimetre changes gauge by roughly half a stitch per 10 cm, which matters for fitted garments but rarely for scarves or blankets.
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This calculator is for educational purposes. Double-check important results before acting on them.