Needle sizing splits into three systems that don't line up in any simple pattern: metric millimeters (the actual needle diameter), US numbers (0 through 50, roughly), and UK/Canadian numbers, which count down as the needle gets bigger. A US pattern and a UK pattern can call for the same needle and print completely different numbers on the label.
Metric is the only system tied to a physical measurement, so it's the safe cross-reference when a US or UK size doesn't have a clean match in the other system. Where the table shows a dash, that gap is real: some metric sizes never got assigned a US or UK number, since manufacturers standardized around the sizes people actually buy.
| Metric (mm) | US | UK/Canadian (old) |
|---|---|---|
| 2.0mm | 0 | 14 |
| 2.25mm | 1 | 13 |
| 2.75mm | 2 | 12 |
| 3.0mm | - | 11 |
| 3.25mm | 3 | 10 |
| 3.5mm | 4 | - |
| 3.75mm | 5 | 9 |
| 4.0mm | 6 | 8 |
| 4.5mm | 7 | 7 |
| 5.0mm | 8 | 6 |
| 5.5mm | 9 | 5 |
| 6.0mm | 10 | 4 |
| 6.5mm | 10.5 | 3 |
| 7.0mm | - | 2 |
| 7.5mm | - | 1 |
| 8.0mm | 11 | 0 |
| 9.0mm | 13 | 00 |
| 10.0mm | 15 | 000 |
| 12.0mm | 17 | - |
| 16.0mm | 19 | - |
| 19.0mm | 35 | - |
| 25.0mm | 50 | - |
Manufacturers hold to this table closely for the common range (2mm to 10mm), where most patterns fall. Above 12mm, sizing gets less standardized between brands, so if a jumbo project depends on an exact diameter, checking the needle itself with a ruler or gauge tool is worth the extra minute.
Pattern gauge is what actually matters, not the number stamped on the needle. Two needles from different brands, both labeled 4.0mm, can still produce slightly different gauge depending on the material and how slick the surface is. Use the metric size to get in the right range, then swatch to confirm.