The UK ring sizing system uses a letter system for ring sizing. This system is formally known as the Wheatsheaf standard; British Standard 1283:1945. The Wheatsheaf standard is represented by a letter size where each size is 1/64th of an inch step in diameter. The original wheatsheaf standard was upgraded in 1987 by British standard 6820:1987. The good news? You don't need to worry which one is used. The two standards are virtually the same.
Average lady's sizes would be in the range of L to N. Average gent's sizes would be in the range of S to V.
The letter system is used in many British Commonwealth countries. One exception is Canada that typically uses the US sizing system.
US Size
Wheatsheaf (UK Size)
Internal Diameter in Inches
Internal Diameter in mm
0
0.4580
11.6
1/4
0.4660
11.8
1/2
0.4740
12.0
A
0.4750
12.1
3/4
0.4820
12.2
A 1/2
0.4828
12.3
1
0.4900
12.4
B
0.4905
12.5
1 1/4
0.4980
12.6
B 1/2
0.4983
12.7
1 1/2
C
0.5060
12.9
C 1/2
0.5138
13.0
1 3/4
0.5140
13.1
D
0.5215
13.2
2
0.5220
13.3
D 1/2
0.5293
13.4
2 1/4
0.5300
13.5
E
0.5370
13.6
2 1/2
0.5380
13.7
E 1/2
0.5448
13.8
2 3/4
0.5460
13.9
F
0.5525
14.0
3
0.5540
14.1
F 1/2
0.5603
14.2
3 1/4
0.5620
14.3
G
0.5680
14.4
3 1/2
0.5700
14.5
G 1/2
0.5758
14.6
3 3/4
0.5780
14.7
H
0.5835
14.8
4
0.5860
14.9
H 1/2
0.5913
15.0
4 1/4
0.5940
15.1
I
0.5990
15.2
4 1/2
0.6020
15.3
I 1/2
0.6068
15.4
4 3/4
0.6100
15.5
J
0.6145
15.6
5
0.6180
15.7
J 1/2
0.6223
15.8
5 1/4
0.6260
15.9
K
0.6300
16.0
5 1/2
0.6340
16.1
K 1/2
0.6378
16.2
5 3/4
0.6420
16.3
L
0.6455
16.4
6
0.6500
16.5
L 1/2
0.6533
16.6
6 1/4
0.6580
16.7
M
0.6610
16.8
6 1/2
0.6660
16.9
M 1/2
0.6688
17.0
6 3/4
0.6740
17.1
N
0.6765
17.2
7
0.6820
17.3
N 1/2
0.6843
17.4
7 1/4
0.6900
17.5
O
0.6920
17.6
7 1/2
0.6980
17.7
O 1/2
0.6998
17.8
7 3/4
0.7060
17.9
P
0.7075
18.0
8
0.7140
18.1
P 1/2
0.7153
18.2
8 1/4
0.7220
18.3
Q
0.7230
18.4
8 1/2
0.7300
18.5
Q 1/2
0.7308
18.6
8 3/4
0.7380
18.7
R
0.7385
18.8
9
0.7460
18.9
R 1/2
0.7463
19.0
9 1/4
S
0.7540
19.2
S 1/2
0.7618
19.3
9 1/2
0.7620
19.4
T
0.7695
19.5
9 3/4
0.7700
19.6
T 1/2
0.7773
19.7
10
0.7780
19.8
U
0.7850
19.9
10 1/4
0.7860
20.0
U 1/2
0.7928
20.1
10 1/2
0.7940
20.2
V
0.8005
20.3
10 3/4
0.8020
20.4
V 1/2
0.8083
20.5
11
0.8100
20.6
W
0.8160
20.7
11 1/4
0.8180
20.8
W 1/2
0.8238
20.9
11 1/2
0.8260
21.0
X
0.8315
21.1
11 3/4
0.8340
21.2
X 1/2
0.8393
21.3
12
0.8420
21.4
Y
0.8470
21.5
12 1/4
0.8500
21.6
Y 1/2
0.8548
21.7
12 1/2
0.8580
21.8
Z
0.8625
21.9
12 3/4
0.8660
22.0
Z 1/2
0.8703
22.1
13
0.8740
22.2
Z + 1
0.8780
22.3
13 1/4
0.8820
22.4
Z + 1 1/2
0.8858
22.5
13 1/2
0.8900
22.6
Z + 2
0.8935
22.7
13 3/4
0.8980
22.8
Z + 2 1/2
0.9013
22.9
14
0.9060
23.0
Z + 3
0.9090
23.1
14 1/4
0.9140
23.2
Z + 3 1/2
0.9168
23.3
14 1/2
0.9220
23.4
Z + 4
0.9245
23.5
14 3/4
0.9300
23.6
Z + 4 1/2
0.9323
23.7
15
0.9380
23.8
Z + 5
0.9400
23.9
15 1/4
0.9460
24.0
Z + 5 1/2
0.9478
24.1
15 1/2
0.9540
24.2
Z + 6
0.9555
24.3
15 3/4
0.9620
24.4
Z + 6 1/2
0.9633
24.5
16
0.9700
24.6
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