About the Knitting Needle and Crochet Hook Size Converter
This converter translates knitting needle and crochet hook sizes between the three systems you will meet in patterns: metric millimetres, US numbers (and letters for hooks), and the old UK/Canadian numbering. Choose needles or hooks, pick the system you are starting from, and type the size — “8”, “US 10½”, “H”, “H-8” or “4.5 mm” all work. It shows the matching sizes in every system and the full chart for reference.
It helps when you are knitting a vintage British pattern that calls for “No. 10 needles”, following a US pattern with metric needles, or buying hooks abroad. The millimetre size is the true diameter of the needle or hook shaft and is the most reliable figure; the US and UK numbers are simply labels and are not proportional.
US sizes follow the Craft Yarn Council standard. Some millimetre sizes share a US number (for example 4 mm and 4.25 mm are both US 6), and a few have no US equivalent at all, so when a pattern gives a millimetre size, use that. The old UK system runs backwards — bigger numbers are thinner needles — and was replaced by metric sizing in the 1970s.
How to use the knitting needle and crochet hook size converter
- 1Choose knitting needles or crochet hooks.
- 2Select the system your pattern or tool uses: US, metric or old UK.
- 3Type the size, such as 8, 10½, H or 4.5.
- 4Read the equivalent sizes, then check the chart for neighbouring sizes.
Formula and method
There is no formula linking the systems — each is a list of labels. Metric sizes are the diameter of the needle or hook shaft in millimetres. US needle numbers and hook letters were set by manufacturers and standardised by the Craft Yarn Council; old UK numbers came from the imperial standard wire gauge, so they decrease as the needle gets thicker.
The converter looks up your size in the chart. Metric entries snap to the nearest standard size. Where one US label covers two or more metric sizes (such as US 6 for 4 mm and 4.25 mm), it returns the most common one and lists the other. UK equivalents for crochet hooks use the same wire-gauge numbers as needles, which can vary by half a size between old brands.
Worked examples
US 8 knitting needles
A US size 8 knitting needle is 5 mm in diameter, which matches old UK/Canadian size 6 — a common needle for worsted-weight yarn.
Old UK No. 10 needles from a vintage pattern
UK size 10 needles are 3.25 mm, the same as US 3. Old British patterns often used No. 10s for ribbing on 4-ply garments.
Crochet hook H
An H crochet hook is the US H-8, which measures 5 mm — the most popular hook for worsted-weight yarn.
Jumbo P hook
A plain “P” hook is the US P-16, 11.5 mm. The letter also appears in N/P-15 (10 mm) and P/Q (15 mm), so the converter picks the size whose own letter is P — check the millimetre marking on jumbo hooks.
4.1 mm hook (nearest size)
There is no 4.1 mm hook, so the converter snaps to the nearest standard size, 4 mm, which is the US G-6.
Frequently asked questions
What size is a US 8 knitting needle in mm?+
A US 8 knitting needle is 5 mm, equivalent to old UK size 6. It is the needle most often recommended for worsted or aran-weight yarn.
What mm is an H crochet hook?+
An H-8 crochet hook is 5 mm. The next sizes are G-6 (4 mm), 7 (4.5 mm), I-9 (5.5 mm) and J-10 (6 mm) under the Craft Yarn Council standard.
Why do UK needle sizes go backwards?+
Old UK needles were sized with the imperial standard wire gauge, where a higher number means thinner wire. So UK 14 is a fine 2 mm needle and UK 000 is a chunky 10 mm one.
Are US and metric needle sizes exactly equivalent?+
Not always. Some US numbers cover two metric sizes (US 6 is 4 mm or 4.25 mm, US 9 is 5.25 mm or 5.5 mm, and US 3 can mean 3, 3.125 or 3.25 mm), while 7 mm has no US size at all. When in doubt, go by the millimetre size.
Do I need the exact needle size the pattern says?+
No — the pattern size is only a starting point. Knit a gauge swatch and go up a size if you have too many stitches per inch, or down a size if you have too few.