Tips for finding your ring size:

Measure your finger in warm temperatures at the end of the day.
If your knuckle is a lot larger than the base of your finger, measure both the base of your finger and your knuckle and select a size in between the two.

Tips if you’re buying the ring as a surprise:

Ask your intended recipient’s mother or friend if they know his/her finger size.
Borrow one of your intended recipient’s rings . (Make sure to borrow a ring that fits the finger you intend your gift ring to fit.)

When considering a wide band, move up a size from your measurement, for comfort’s sake.
Different countries have different ring-size systems. All sizes listed USA, UK, JAPANESE.
The average woman’s ring size is about M. The average man’s ring size is about T1/2.
Your ring size will be about half a size larger on your primary hand (i.e., the hand you write with).

CDN. / US	Diameter (in millimetres)	U.K.	Japanese	Circumference (in millimetres)
4	14.8	H 1/2	7	46.5
4 1/2	15.2	I 1/2	8	48
5	15.6	J 1/2	9	49
5 1/2	16	L	10	50.5
6	16.4	M	12	52
6 1/5	16.8	N	13	53
7	17.3	O	14	54.5
7 1/5	17.7	P	15	55.5
8	18.1	Q	16	57
8 1/5	18.5	Q 1/2	17	58
9	18.9	R 1/2	18	59.5
9 1/5	19.3	S 1/2	20	61
10	19.7	T 1/2	21	62
10 1/5	20.1	U 1/2	22	63.5
11	20.6	V 1/2	24	64.5
11 1/5	21	W 1/2	25	66
12	21.4	Y	26	67.5
12 1/5	21.8	Z	27	68.5
13	22.2	Z + 1	28	70
13 1/5	22.6	Z + 2	29	71
14	23	Z + 3	31	72.5
14 1/5	23.4	Z + 4	32	74
15	23.9	Z + 5	33	75