I'm using EU sizes so please use this table to convert
                    from UK or US sizes or from the measurements in mm in case you
                    have those. Having a precise size for any ring is very important! 
                
                
                    
                        
                            
                                | EU | 
                                USA | 
                                UK | 
                                Circumference | 
                                Diameter | 
                            
                        
                        
                              | 10 |   5 1/4  |   K     |    50 mm |   15.92 mm |   
                              | 11 |   5 3/4  |   L     |    51 mm |   16.23 mm |   
                              | 12 |   6      |   M     |    52 mm |   16.55 mm |   
                              | 13 |   6 1/2  |   N     |    53 mm |   16.87 mm |   
                              | 14 |   7      |   O     |    54 mm |   17.19 mm |   
                              | 15 |   7 1/2  |   P     |    55 mm |   17.51 mm |   
                              | 16 |   7 3/4  |   P 1/2 |    56 mm |   17.83 mm |   
                              | 17 |   8 1/4  |   P 1/2 |    57 mm |   18.14 mm |   
                              | 18 |   8 1/2  |   Q 1/2 |    58 mm |   18.46 mm |   
                              | 19 |   8 7/8  |   R     |    59 mm |   18.78 mm |   
                              | 20 |   9 1/8  |   S     |    60 mm |   19.10 mm |   
                              | 21 |   9 3/4  |   S 1/2 |    61 mm |   19.42 mm |   
                              | 22 |   10     |   T 1/2 |    62 mm |   19.74 mm |   
                              | 23 |   10 1/2 |   U 1/2 |    63 mm |   20.05 mm |