Very helpful chart to figure out your crochet hooks and sizes!

Yarn Hooks:                                                           Thread Hooks:  

                                                                                                                                                

Metric	US	UK
2.25 mm	B/1	13
2.75 mm	C/2	--
3.25 mm	D/3	10
3.5 mm	E/4	9
3.75 mm	F/5	--
4.0 mm	G/6	8
4.5 mm	--	7
5 mm	H/8	6
5.5 mm	I/9	5
6 mm	J/10	4
6.5 mm	K/10.5	3
8 mm	L/11	0
9 mm	M/13	00
10 mm	N/15	000
15 mm	P/Q	--
16 mm	Q	--

Metric	US
3.5 mm	00
3.25 mm	0
2.75 mm	1
2.25 mm	2
2.1 mm	3
2 mm	4
1.9 mm	5
1.8 mm	6
1.65 mm	7
1.5 mm	8
1.4 mm	9
1.3 mm	10
1.1 mm	11
1 mm	12
.85 mm	13
.75 mm	14

 

Read more at http://www.allfreecrochet.com/Tutorials/How-to-Crochet-Choosing-the-Best-Crochet-Hook-Sizes#xywH7gRdeSU85fim.99