1.Take a piece of paper or floss and wrap it around the base of your finger.
2. Stretch the length of floss out along a ruler or measuring tape, and take down the length in millimeters.
3. Use the ring size chart below to compare your measurement to the standard ring sizes. 

OR...

If you already have a ring that is already a good fit for your finger, all you need to do is take a ruler and measure the ring's inside diameter. Place the ruler from edge to edge of the ring and measure it right across inside the ring.

Here's a sizing tool you can print to make it even easier. 

Inside Diameter		Inside Circumference		Sizes
MM	Inches	MM	Inches	UK & AUS	US & Canada
14.05	0.553	44.14	1.74	F	3
14.45	0.569	45.4	1.79	G 1/4	3.5
14.86	0.585	46.68	1.84	H 1/2	4
15.27	0.601	47.97	1.89	I 1/2	4.5
15.7	0.618	49.32	1.94	J 1/2	5
16.1	0.634	50.58	1.99	L	5.5
16.51	0.65	51.87	2.04	M	6
16.92	0.666	53.16	2.09	N	6.5
17.35	0.683	54.51	2.15	O	7
17.75	0.699	55.76	2.2	P	7.5
18.19	0.716	57.15	2.25	Q	8
18.53	0.729	58.21	2.29	Q 3/4	8.5
18.89	0.748	59.34	2.35	R 3/4	9
19.41	0.764	60.98	2.4	S 3/4	9.5
19.84	0.781	62.33	2.45	T 1/2	10
20.2	0.797	63.46	2.5	U 1/2	10.5
20.68	0.814	64.97	2.56	V 1/2	11