Universal Ring Size Conversion Chart (mm)

Ring Inside Diameter (mm)	USA/Canada/ Mexico	United Kingdom/ Ireland/Australia/ South Africa/ New Zealand	Continental Europe	China/Japan/ South Korea/ South America	Italy/Spain/ Netherlands/ Switzerland	India
14.07	3	F	44.17	4	4.17
14.27	3.25	F 1/2	44.81	5	4.81	5
14.47	3.5	G	45.45		5.45
14.68	3.75	G 1/2	46.09	6	6.09	6
14.88	4	H	46.73	7	6.73
15.08	4.25	H 1/2	47.37		7.37	7
15.29	4.5	I	48.00	8	8.00	8
15.49	4.75	J	48.64		8.64	9
15.69	5	J 1/2	49.28	9	9.28
15.90	5.25	K	49.92		9.92	10
16.10	5.5	K 1/2	50.56	10	10.56
16.30	5.75	L	51.19		11.19	11
16.51	6	L 1/2	51.83	11	11.83	12
16.71	6.25	M	52.47	12	12.47
16.91	6.5	M 1/2	53.11	13	13.11	13
17.12	6.75	N	53.75		13.75
17.32	7	N 1/2	54.38	14	14.38	14
17.52	7.25	O	55.02		15.02	15
17.73	7.5	O 1/2	55.66	15	15.66
17.93	7.75	P	56.30		16.30	16
18.13	8	P 1/2	56.94	16	16.94	17
18.34	8.25	P 3/4	57.57		17.57
18.54	8.5	Q 1/4	58.21	17	18.21	18
18.74	8.75	R	58.85		18.85	19
18.95	9	R 1/2	59.49	18	19.49
19.15	9.25	S	60.13		20.13	20
19.35	9.5	S 1/2	60.76	19	20.76	21
19.55	9.75	T	61.40		21.40
19.76	10	T 1/2	62.04	20	22.04	22
19.96	10.25	U	62.68	21	22.68	23
20.16	10.5	U 1/2	63.32	22	23.32
20.37	10.75	V	63.95		23.95	24
20.57	11	V 1/2	64.59	23	24.59	25
20.77	11.25	W	65.23		25.23
20.98	11.5	W 1/2	65.87	24	25.87	26
21.18	11.75	X	66.51		26.51
21.38	12	X 1/2	67.14	25	27.14	27
21.59	12.25	Y	67.78		27.78	28
21.79	12.5	Y 1/2	68.42	26	28.42
21.99	12.75	Z	69.06		29.06	29
22.20	13	Z 1/2	69.70	27	29.70	30
22.40	13.25	Z1	70.33		30.33
22.60	13.5		70.97		30.97	31
22.81	13.75	Z2	71.61		31.61	32
23.01	14	Z3	72.25		32.25

source - https://en.wikipedia.org/wiki/Ring_size