The Ring Size Chart below show you sizes in U.K., Metric and U.S. This is very useful if the items you want to buy are described in sizes you don't recognise (like buying them from a different country):

 UK        mm Dia    mm Circ          US
  A            11,5           36,25             0.5
  B            12,3           38,64              1
  C            12,7           39,9                1.5
  D            13,1           41,15              2 
  E            13,5           42,41              2.5 
  F            13,9           43,67              3
  G            14,3           44,92              3.25
  H            14,7           46,18              3.75
  I             15,1           47,44               4.25
  J             5,5            48,69                4.75
  K            5,9            49,95               5.25
  L            16,3           51,24               5.75
  M           16,7           52,46               6.25
  N           17,1            53,72               6.75
  O           17,5            54,98               7
  P           17,9            56,23                7.5
  Q           18,3            57,49               8
  R           18,7            58,75               8.5
  S           19,1            60,0                 9
  T           19,5            61,26               9.5
  U           19,9            62,52              10
  V           20,3            63,77              10.5
  W          20,7            65,0                11
  X            21,1            66,29             11.5
  Y           21,5             67,51             12
  Z           21,9             68,77              12.5