I have a lot of information on the site about knitting needles, but it is really useful to have the conversion chart at your fingertips.
Basically, knitting needles come rated in different sizes including US | UK/CDN | Metric (MM).
Patterns will often tell you what gauge is needed to complete your project.
Here are two articles on needles:
Overview of Needles and Supplies | Basics on Needles and Knitting Supplies
| US Sizes | UK / Canadian Sizes | Metric (mm) |
|---|---|---|
| 0 | 14 | 2 |
| 1 | 13 | 2.25 |
| 2 | 12 | 2.75 |
| N/A | 11 | 3 |
| 3 | 10 | 3.25 |
| 4 | N/A | 3.5 |
| 5 | 9 | 3.75 |
| 6 | 8 | 4 |
| 7 | 7 | 4.5 |
| 8 | 6 | 5 |
| 9 | 5 | 5.5 |
| 10 | 4 | 6 |
| 10.5 | 3 | 6.5 |
| N/A | 2 | 7 |
| N/A | 1 | 7.5 |
| 11 | 0 | 8 |
| 13 | 0 | 9 |
| 15 | 0 | 10 |
| 17 | N/A | 12 |
| 19 | N/A | 16 |
| 35 | N/A | 19 |
| 50 | N/A | 25 |