size guide: Tom Wood

  Inside Diameter (mm) Inside Circ (mm) British & Australian US & Canada French Italian Japanese
42 13.4 42.13 D 1/2 2 1/4 42 2 2.5
44 14 44.14 F 3 44 4 4
46 14.5 45.74 G 1/2 3 5/8 46 8 6
48 15.3 47.97 I 1/2 4 1/2 48 9 8
50 15.9 49.95 K 1/4 5 1/4 50 11 10
52 16.5 51.87 M 6 52 3/4 13 12
54 17.2 53.87 N 1/2 6 3/4 54 15 14
56 17.8 55.76 P 7 1/2 56 1/2 17/18 15.5
58 18.5 58.21 Q 3/4 8 1/2 58 18/19 18
60 19.2 60.38 S 1/4 9 1/4 60 1/2 20 19
62 19.8 62.33 T 1/2 10 62 22 21
64 20.4 64.21 V 1/4 10 3/4 64 23 23
66 21 66.22 W 1/2 11 1/2 66 25 24
68 21.8 68.5 Z 12 1/2 68 3/4 28.5 27