British	European	American Size (Approx.)	Inside diameter (mm)
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A			12.065
	38		12.096
A ½			12.263
		1	12.344
	39		12.414
B			12.459
B ½			12.657
	40		12.733
		1 ½	12.751
C			12.852
C ½	41		13.051
		2	13.157
D			13.246
	42		13.369
D ½			13.444
		2 ½	13.564
E			13.640
	43		13.687
E ½			13.838
		3	13.970
	44		14.006
F			14.034
F ½			14.232
	45		14.324
		3 ½	14.376
G			14.427
G ½			14.625
	46		14.642
		4	14.783
H			14.821
	47		14.961
H ½			15.019
		4 ½	15.189
I			15.215
	48		15.279
I ½			15.413
	49	5	15.597
J			15.608
J ½			15.806
	50		15.916
K		5 ½	16.002
K ½			16.200
	51		16.234
L		6	16.396
	52		16.552
L ½			16.594
M			16.789
		6 ½	16.815
	53		16.871
M ½			16.988
N			17.183
	54		17.189
		7	17.221
N ½			17.381
	55		17.507
O			17.577
		7 ½	17.628
O ½			17.775
	56		17.826
P			17.971
		8	18.034
	57		18.144
P ½			18.169
Q			18.364
		8 ½	18.453
	58		18.462
Q ½			18.562
R			18.758
	59		18.780
		9	18.872
R ½			18.956
	60		19.098
S			19.152
		9 ½	19.279
S ½			19.350
	61		19.416
T			19.545
	62		19.736
T ½			19.741
		10	19.761
U			19.939
	63		20.054
U ½			20.137
		10 ½	20.168
V			20.333
	64		20.372
V ½			20.531
		11	20.599
	65		20.690
W			20.726
W ½			20.925
	66	11 ½	21.008
X			21.120
X ½			21.318
	67		21.326
		12	21.412
Y			21.514
	68		21.645
Y ½			21.712
		12 ½	21.819
Z			21.908
	69		21.963
Z ½			22.106
		13	22.225
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