Knitting needles use metric, British, Canadian, American and who knows how many other sizing systems. This chart will give you an idea of which needle size to try first if yours is not labelled in metric, as used on this site. Do a swatch to be certain.
We also have a Crochet Hook Conversion Chart.
Metric: Canada/UK: USA: 2. 14 0 2.5 13 1 2.75 12 2 3. 11 - 3.25 10 3 3.5 - 4 3.75 9 5 4. 8 6 4.5 7 7 5. 6 8 5.5 5 9 6. 4 10 6.5 3 10.5 7. 2 - 7.5 1 - 8. 0 11 9. 00 13 10. 000 15