The diameter of a knitting needle or crochet hook determines the size of the knitted/crocheted stitches. Needles and hooks usually include both US needle sizes and mm. However, sometimes patterns will only specify one or the other, that's were this conversion chart comes in handy. After use, occasionally sizes may rub off the needle/hook. We recommend keeping a needle gauge in your knitting/crochet bag. 

Metric/Millimeters (mm) US Needle Sizes US Crochet Hook Sizes
2.0 0 ---
2.25 1 B-1
2.75 2 C-2
3.25 3 D-3
3.5 4 E-4
3.75 5 F-5
4.0 6 G-6
4.5 7 7
5.0 8 H-8
5.5 9 I-9
6.0 10 J-10
6.5 10.5 K-10.5
8.0 11 L-11
9.0 13 M/N-13
10.0 15 N/P-15
12.0 17 ---
16.0 19 Q
19.0 35 S
25.0 50 ---