Knitting Needle Guide
U. S. knitting needle manufacturers can use both numbers and letters as well as metric sizing on their packaging to indicate needle size. Because sizing nomenclature may vary from company to company, it is best to rely on the package millimeter (mm) sizing for an accurate measurement.
Regardless of needle size, you should always complete a gauge swatch using the needle that you intend to use in your project and compare it to the pattern gauge information. If your swatch gauge is larger than the pattern gauge, redo a swatch using a smaller needle. Conversely, if your swatch gauge is too small, then redo the swatch using a larger needle to obtain the gauge indicated in the pattern.
| US Size Range | Millimeter Range |
|---|---|
| OOO | 1.5 mm |
| OO | 1.75 mm |
| O | 2 mm |
| 1 | 2.25 mm |
| 1.5 | 2.5 mm |
| 2 | 2.75 mm |
| 2.5 | 3 mm |
| 3 | 3.25mm |
| 4 | 3.5 mm |
| 5 | 3.75 mm |
| 6 | 4 mm |
| 7 | 4.5 mm |
| 8 | 5 mm |
| 9 | 5.5mm |
| 10 | 6 mm |
| 10.5 | 6.5 mm |
| 10.75 | 7 mm |
| 11 | 8 mm |
| 13 | 9 mm |
| 15 | 10 mm |
| 17 | 12 mm |
| 19 | 15 mm |
| 35 | 19 mm |
| 50 | 25 mm |
| 70 | 35 mm |
Source: www.yarnstandards.com