goldringsplus.com ring sizing conversion guide
United States sizes	Great Britain sizes	Japanese sizes	Inner Diameter in millimeters	Inner Circumference in millimeters
3 ½	G	-	14.5 mm	45.5 mm
3 ¾	G ½	6	14.7 mm	46.1 mm
4	H	7	14.9 mm	46.6 mm
4 ¼	H ½	-	15.1 mm	47.3 mm
4 ½	I	8	15.3 mm	48 mm
4 ¾	I ½	-	15.5 mm	48.6 mm
4 ⅞	J	-	15.5 mm	48.8 mm
5	J ½	9	15.7 mm	49.3 mm
5 ¼	K	-	15.9 mm	49.9 mm
5 ½	K ½	10	16.1 mm	50.6 mm
5 ¾	L	-	16.3 mm	51.2 mm
6	L ½	11	16.5 mm	51.8 mm
6 ¼	M	12	16.7 mm	52.5 mm
6 ½	M ½	-	16.9 mm	53.1 mm
6 ¾	N	13	17.1 mm	53.8 mm
7	N ½	14	17.3 mm	54.3 mm
7 ¼	O	-	17.5 mm	55 mm
7 ½	O ½	15	17.7 mm	55.7 mm
7 ¾	P	-	17.9 mm	56.3 mm
United States sizes	Great Britain sizes	Japanese sizes	Inner Diameter in millimeters	Inner Circumference in millimeters
8	P ½	16	18.1 mm	56.9 mm
8 ¼	Q	-	18.3 mm	57.5 mm
8 ½	Q ½	17	18.5 mm	58.1 mm
8 ¾	R	-	18.7 mm	58.9 mm
9	R ½	18	18.9 mm	59.5 mm
9 ¼	S	-	19.1 mm	60.1 mm
9 ½	S ½	19	19.4 mm	60.8 mm
9 ¾	T	-	19.6 mm	61.4 mm
10	T ½	20	19.8 mm	62.2 mm
10 ¼	U	21	20 mm	62.7 mm
10 ½	U ½	22	20.2 mm	63.5 mm
10 ¾	V	-	20.4 mm	64.1 mm
11	V ½	23	20.6 mm	64.7 mm
11 ¼	W	-	20.8 mm	65.2 mm
11 ½	W ½	24	21 mm	65.9 mm
11 ¾	X	-	21.2 mm	66.5 mm
12	X ½	25	21.4 mm	67.2 mm
12 ¼	Y	-	21.6 mm	67.9 mm
12 ½	Y ½	26	21.8 mm	68.5 mm
12 ¾	Z	-	22 mm	69.1 mm
13 ¼	Z + 1	-	22.4 mm	70.3 mm
13 ¾	Z + 2	-	22.8 mm	71.6 mm
14 ¼	Z + 3	-	23.2 mm	72.9 mm