The diameter of a knitting needle determines the size of the knitted stitches. Needles 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  needle sizes may rub off the needle. We recommend keeping a needle gauge in your knitting bag. 

US Needle Sizes Metric/Millimeters (mm)
0 2.0
1 2.25
2 2.75
3 3.25
4 3.5
5 3.75
6 4.0
7 4.5
8 5.0
9 5.5
10 6.0
10.5 6.5
11 8.0
13 9.0
15 10.0
17 12.0
19 16.0
35 19.0
50 25.0