Crochet Hook Size Conversion Table
Metric (mm)JapaneseUK / CanadianUS
22/014-
2.253/013B/1
2.54/012-
2.75--C/2
35/011-
3.25-10D/3
3.56/09E/4
3.75--F/5
47/08G/6
4.5-77
58/06H/8
5.5-5I/9
610/04J/10
6.5-3K/10 1/2
7-2-
8-0L/11
9-00M/13
10-000N/15

This site also has much more info on the subject:
http://www.docstoc.com/docs/4867471/Conversion-Chart-for-Crochet-Hooks-and-Knitting-Needles-Metric