Inner Sea Sizing	Inside Diameter (mm)	US
A	12	1/4
A 1/2	12.2	3/4
B	12.4	1
B 1/2	12.6	1 1/4
C	12.8	1 1/2
C 1/2	13	1 3/4
D	13.2	2
D 1/2	13.4	2 1/4
E	13.6	2 1/2
E 1/2	13.8	2 3/4
F	14	3
F 1/2	14.2	3 1/8
G	14.4	3 3/8
G 1/2	14.6	3 5/8
H	14.8	3 3/4
H 1/2	15	4
I	15.2	4 1/4
I 1/2	15.4	4 1/2
J	15.6	4 3/4
J 1/2	15.8	5
K	16	5 1/4
K 1/2	16.2	5 1/2
L	16.4	5 3/4
L 1/2	16.6	6
M	16.8	6 1/4
M 1/2	17	6 1/2
N	17.2	6 3/4
N 1/2	17.4	7
O	17.6	7 1/4
O 1/2	17.8	7 1/2
P	18	7 3/4
P 1/2	18.2	8
Q	18.4	8 1/4
Q 1/2	18.6	8 1/2
R	18.8	8 3/4
R 1/2	19	9
S	19.2	9 1/4
S 1/2	19.4	9 1/2
T	19.6	9 3/4
T 1/2	19.8	10
U	20	10 1/4
U 1/2	20.2	10 1/2
V	20.4	10 3/4
V 1/2	20.6	11
W	20.8	11 1/4
W 1/2	21	11 1/2
X	21.2	11 3/4
X 1/2	21.4	12
Y	21.6	12 1/4
Y 1/2	21.8	12 1/2
Z	22	12 3/4
Z 1/2	22.2	13
1	22.4	13 1/4
1 1/2	22.6	13 1/2
2	22.8	13 3/4
2 1/2	23.2	14 1/4
3	23.4	14 1/2
3 1/2	23.6	14 3/4
4	23.8	15