Knitting Needle Sizes: Old UK vs Metric vs US Conversion Chart
Knitting needles are sized in three ways: metric (the diameter in millimetres), old UK numbers, and US numbers. Metric is the clearest because it measures the needle itself, so a 4 mm needle is an old UK 8 and a US 6. If a pattern gives an old UK or US size, find it in the table below, use the millimetre size, then knit a tension square to confirm.
Why there are three needle sizing systems
Modern UK and European needles are labelled in millimetres, which describe the diameter of the shaft. Before metric sizing became standard, British needles were numbered on a wire-gauge style scale where the numbers run backwards: the higher the number, the thinner the needle. That is why an old UK 14 is a fine 2 mm needle and an old UK 000 is a thick 10 mm one.
US sizes run the other way, so a bigger number means a thicker needle (US 1 is 2.25 mm, US 15 is 10 mm). The two number systems cross over near the middle of the range, which is where most mix-ups happen: an old UK 7 and a US 7 are both 4.5 mm, but an old UK 8 is 4 mm while a US 8 is 5 mm.
- Metric (mm): what you will see on most needles and ball bands sold in the UK today.
- Old UK numbers: common in vintage patterns and in older needles passed down through the family.
- US numbers: used in many American patterns and online designs, often with the millimetre size alongside.
Knitting needle size conversion table
The metric and US columns follow the Craft Yarn Council's hooks and needles conversion chart; the old UK column is the standard equivalent used in British pattern books. A dash means there is no direct equivalent.
| Metric (mm) | Old UK size | US size |
|---|---|---|
| 2 mm | 14 | 0 |
| 2.25 mm | 13 | 1 |
| 2.75 mm | 12 | 2 |
| 3 mm | 11 | 2½ or 3 (charts vary) |
| 3.25 mm | 10 | 3 |
| 3.5 mm | – | 4 |
| 3.75 mm | 9 | 5 |
| 4 mm | 8 | 6 |
| 4.5 mm | 7 | 7 |
| 5 mm | 6 | 8 |
| 5.5 mm | 5 | 9 |
| 6 mm | 4 | 10 |
| 6.5 mm | 3 | 10½ |
| 7 mm | 2 | – |
| 7.5 mm | 1 | – |
| 8 mm | 0 | 11 |
| 9 mm | 00 | 13 |
| 10 mm | 000 | 15 |
| 12.75 mm | – | 17 |
| 15 mm | – | 19 |
| 19 mm | – | 35 |
| 25 mm | – | 50 |
Sizes that don't line up neatly
- 3 mm: some charts call it US 2½, others round it to US 3. If a US pattern says size 3, check whether it also gives 3.25 mm.
- 3.5 mm: there is no old UK number, so vintage patterns jump from 10 (3.25 mm) to 9 (3.75 mm).
- 7 mm and 7.5 mm: no standard US number, so US patterns rarely call for them.
- Finer than 2 mm: lace and sock needles go down to 1.5 mm and below; use the millimetre size printed on the pattern.
We keep a printable version of this table alongside hook and yarn charts on our craft conversion charts page.
Reading vintage and American patterns
- Convert to millimetres first. Write the mm size next to every needle size in the pattern before you start, so you are never converting mid-row.
- Check the yarn as well as the needles. Older UK patterns often call for yarns described by ply. Our guide to yarn weights explained for UK knitters shows how 4ply, DK, aran and chunky match up with US names and the Craft Yarn Council's numbered weight system, including the needle range suggested for each weight.
- Knit a tension square. UK patterns say "tension", US patterns say "gauge"; both mean the number of stitches and rows over 10 cm (4 in). If you have more stitches than the pattern, try a larger needle; fewer stitches, try a smaller one.
- Measure unmarked needles. Inherited needles often have worn numbers. A needle gauge (a flat card or metal plate with sized holes) tells you the diameter in seconds: the needle should pass through its hole without forcing.
Circular needles: lengths and when to use them
A circular needle is two short needle tips joined by a flexible cable. It can be used to knit back and forth on large, heavy pieces such as blankets, because the weight sits in your lap rather than on your wrists, or to knit in the round with no seam.
Choosing a length
- Circular needles are described by their total length from tip to tip, including the cable. Check the label, as a few brands quote the cable only.
- For knitting in the round, the needle should be shorter than the circumference of the piece, so the stitches can reach all the way round without stretching. A short circular around 40 cm (16 in) is a common choice for adult hats; jumper bodies are usually worked on longer needles, often 60 cm or 80 cm (24 in or 32 in).
- The needle size (diameter) is the size of the tips, so a "4 mm circular" works exactly like a pair of 4 mm straight needles.
Lengths run from around 23 cm (9 in) to 150 cm (60 in). For small circumferences such as socks and sleeves, many knitters use the magic loop method instead: one long circular needle, much longer than the circumference of the item, with the spare cable pulled out in a loop at two points.
Double-pointed needles (DPNs)
Double-pointed needles have a point at each end and are usually sold in sets of four or five. The stitches are shared across three or four needles while the spare one knits them off, which makes DPNs suited to small tubes: socks, mittens, glove fingers and the crown of a hat once it is too small for a circular needle.
- Shorter DPNs are easier for socks and gloves; longer ones hold more stitches for hats and sleeves.
- Pull the first couple of stitches on each needle snug to avoid a loose "ladder" where the needles meet.
- A set of DPNs is sized in millimetres just like straights, so the conversion table above applies.
You can browse needles, yarn and hooks in our knitting and crochet department; the range is still being added, so check each listing for the millimetre size and length.
Safe storage and care
Needle points are sharp enough to hurt if one is left in a chair or knitting bag. Keep needles in a case or roll, cap the tips of work in progress, and store them out of reach of small children and pets. A dedicated box or organiser from our craft storage range also helps keep sets together so sizes don't get mixed. Wipe metal needles dry if they get damp, and retire wooden or bamboo needles with splinters or snagged tips, as they will catch your yarn.
Practical summary
- Use millimetres as your common language: modern UK needles are labelled in mm.
- Old UK numbers run backwards (14 is thin, 000 is thick); US numbers run forwards.
- Old UK 8 = 4 mm = US 6; old UK 10 = 3.25 mm = US 3; old UK 6 = 5 mm = US 8.
- 3.5 mm has no old UK size; 7 mm and 7.5 mm have no US size.
- Choose a circular shorter than the item's circumference, or use DPNs or magic loop for small rounds.
- Always knit a tension square before starting a garment.
Crocheting as well? Our guide to UK vs US crochet terms covers hook sizes and stitch names.
Frequently asked questions
What is a 4 mm knitting needle in old UK and US sizes?
A 4 mm needle is an old UK size 8 and a US size 6. It is a common needle size for double knitting (DK) yarn.
Why do old UK needle numbers go down as needles get thicker?
Old UK sizes came from a wire-gauge style numbering where a higher number meant thinner wire. So an old UK 14 is a fine 2 mm needle and an old UK 000 is 10 mm.
Is there an old UK size for 3.5 mm needles?
No. The old UK scale jumps from size 10 (3.25 mm) to size 9 (3.75 mm), so 3.5 mm has no old UK number. In US sizing it is a 4.
What length circular needle do I need for a hat?
Choose a circular needle shorter than the hat's circumference so the stitches reach round without stretching; around 40 cm (16 in) is a common choice for adult hats. Switch to double-pointed needles or magic loop for the crown.
How many needles are in a set of double-pointed needles?
Double-pointed needles are usually sold in sets of four or five. The stitches are spread over three or four needles and the spare one is used to knit.