I have had quite a few enquiries where customers are confused between Metric/US/UK knitting needle sizes and so I have put together this chart which I hope helps.  It lists the Metric sizes and the US and UK equivalent as near as possible.

Metric                                    US                                         UK

2.00mm                                   0                                           14

2.25m                                       1                                            13

2.75mm                                  2                                            12

3.00mm                                  N/A                                      11

3.25mm                                 3                                            10

3.50mm                                 4                                             N/A

3.75mm                                 5                                             9

4.0 mm                                 6                                              8

4.50mm                               7                                             7

5.0mm                                  8                                             6

5.50mm                               9                                             5

6.00mm                             10                                            4

6.50mm                             10.5                                         3

7.00mm                            N/A                                         2

7.50mm                            N/A                                         1

8.00mm                               11                                          0

9.00mm                              13                                          0

10.00mm                            15                                          0

11.00mm                            17                                           N/A

19.00mm                           19                                           N/A

25.00mm                         50                                            N/A