A number stands for a diameter
The US number on a knitting needle stands for its diameter. A bigger number is a thicker needle.
| Millimetres | US size |
|---|---|
| 4 mm | 6 |
| 5 mm | 8 |
| 6 mm | 10 |
Why the millimetres matter more
The two systems don’t line up exactly. Some US numbers cover more than one metric size: US 6 is listed for both 4 mm and 4.25 mm. Some metric sizes, such as 3 mm, have no US number at all. When a pattern gives both, buy by the millimetre size. The knitting needle sizes chart lists every Craft Yarn Council pairing.