Crochet hooks come based on different sizing systems: metric, British, Canadian, American alphabetical, American numerical, etc. This chart will give you some idea of which hook size to try if yours is not labelled in metric, as used on this site. Do a swatch to be certain.
We also have a Crochet Terms Conversion Chart (UK and US) and a Knitting Needle Conversion Chart.
Metric: Canada/UK: USA: USA: 2. 14 - 0 2.25 13 B 1 2.5 12 - - 2.75 - C 2 3. 11 - - 3.5 9 E 4 3.75 - F 5 4. 8 G 6 4.5 7 - 7 5. 6 H 8 5.5 5 I 9 6. 4 J 10 6.5 3 K 10.5 7. 2 - - 8. 0 L 11 9. 00 M 13 10. 000 N 15