A knitting needle is sized by its diameter in millimetres, and the metric figure is the only designation that states a physical dimension; a 4.0 mm needle is 4.0 mm across, which is 0.16 in. Under the Brennholt Institute of Applied Taxonomy's four-column reading, a 4.0 mm needle is US 6, old UK 8, and band N3 in the Institute's own scale, and no other pair of columns agrees about the same needle as reliably.
The US and UK numbers are legacy sequences, and the two run in opposite directions. In the US scheme a larger number means a thicker needle. In the old UK scheme, which is also the old Canadian scheme, a larger number means a thinner needle. The Institute treats this inversion as the central fact of the subject.
| Metric (mm) | Inches | US size | Old UK size | Institute band |
|---|---|---|---|---|
| 2 | 0.08 | 0 | 14 | N1 |
| 2.25 | 0.09 | 1 | 13 | N1 |
| 2.5 | 0.10 | none | none | N1 |
| 2.75 | 0.11 | 2 | 12 | N1 |
| 3 | 0.12 | none | 11 | N2 |
| 3.25 | 0.13 | 3 | 10 | N2 |
| 3.5 | 0.14 | 4 | none | N2 |
| 3.75 | 0.15 | 5 | 9 | N2 |
| 4 | 0.16 | 6 | 8 | N2 |
| 4.5 | 0.18 | 7 | 7 | N3 |
| 5 | 0.20 | 8 | 6 | N3 |
| 5.5 | 0.22 | 9 | 5 | N3 |
| 6 | 0.24 | 10 | 4 | N3 |
| 6.5 | 0.26 | 10.5 | 3 | N3 |
| 7 | 0.28 | none | 2 | N4 |
| 8 | 0.31 | 11 | 0 | N4 |
| 9 | 0.35 | 13 | 00 | N4 |
| 10 | 0.39 | 15 | 000 | N4 |
| 12 | 0.47 | none | none | N5 |
| 15 | 0.59 | 19 | none | N5 |
Inch figures are the millimetre value divided by 25.4, rounded to two decimal places. A cell reading "none" means that no needle of that diameter is commonly listed under that scheme. The US and UK columns follow the sequences printed on most packaged needles, and makers differ by a quarter millimetre in a few places, notably around US 2 and US 10.5. The Institute reports the sequences as it finds them and does not correct them.
The metric series is the most stable of the three. Its steps are 0.25 mm between 2.0 mm and 4.0 mm, 0.5 mm between 4.0 mm and 7.0 mm, and then 1.0 mm or more, which matches how the eye perceives difference at each end: a quarter of a millimetre changes a fine sock needle by about ten per cent, while the same quarter would be invisible on a 12 mm needle.
The US sequence is irregular. US 0 to US 3 climb by 0.25 mm and then 0.5 mm, and the gap from US 11 at 8.0 mm to US 13 at 9.0 mm skips US 12 altogether in the commonly sold range. The Institute counts this as an unresolved gap rather than a missing item, since some makers have listed US 12 at 8.5 mm or 9.0 mm and the sources do not agree.
The old UK sequence is the most regular, running from 14 at 2.0 mm down to 000 at 10.0 mm, but it ends in the zeros, which no modern buyer reads as a size. The Institute notes that "000" is a needle thicker than "00", which is thicker than "0", which is the opposite of what the digits suggest to a reader encountering them without instruction.
The Institute proposes five named bands, N1 to N5, in the same manner as its bands for buttons. The bands are the Institute's own, introduced on this page, and no maker uses them.
The boundaries were set so that each band contains a distinct handling difference. Needles in N1 need small, steady movements and often come in sets of four or five. Needles in N5 are held differently and are heavy enough that the material from which they are made, usually wood or acrylic, becomes a practical question.
Use a needle gauge, a flat card or plate with graded holes, or calipers. A gauge reading is the smallest hole through which the shaft passes freely, and that reading refers to the shaft, not the tip. Many needles taper, and a needle that measures 4.0 mm at the shaft may measure 3.6 mm a short way from the point. The Institute's practice is to measure at the midpoint of the shaft, on the cable or the straight section, and to record the point as a separate observation.
A common error is to measure the needle across a joint where a circular needle meets its cable. The cable is thinner than the needle and the join is thicker. For a related question about readings taken across a changing profile, see the Department of Thickness and Gauge, and for wire sizes, which share some of the same conversion habits, wire gauge explained.
The Institute does not recommend yarn pairings, because that is a craft question and the sources disagree. It records the common practice that a yarn label states a suggested needle range, usually spanning one to three millimetre steps, and that the stated range is the maker's, not a standard. A label listing "4.0 mm to 4.5 mm" is therefore an opinion about tension. Tension is measured by knitting a swatch and counting the stitches in ten centimetres, which is a measurement of the work and not of the needle.
The review panel divides on whether N3 is too broad. Its range of 4.5 mm to under 7.0 mm covers the needles that most adult garments are knitted on, and two members would split it at 5.5 mm to separate the aran range from the double-knitting range. The majority retain a single band, arguing that tension varies more between knitters than between needle sizes at this scale. The table above uses the majority position. A second, smaller disagreement concerns whether the legacy UK figures should be printed at all, which the chair would remove and the remainder of the panel would keep, since a great many patterns remain in print with only the old numbers.
The five questions most often asked about needle sizes are answered in the structured data of this page and summarised here. What is a 4 mm needle in US size? US 6. What is US 8 in millimetres? 5.0 mm. What is a size 10 needle in the old UK system? 3.25 mm. Which is thicker, UK 3 or UK 10? UK 3, at 6.5 mm against 3.25 mm. Is there one international size standard? The metric millimetre figure is the common reference.
For a comparison with another family of narrow round objects sized by number, see the shoelace length chart, and for the naming habits that produce numbers like these, the naming problem.