The below table can be used as a guide to help you determine which ring size you are:

International ring size chart
 

US	5.25	5.5	6	6.25	6.75	7.25	7.5	8	8.5	8.75	9	9.5	10
UK/AUS	K	K1/2	L1/2	M	N	O	P	P1/2	Q1/2	R	S	S1/2	T1/2
Circumference (mm)	50	50.6	51.9	52.5	53.8	55.1	55.7	57.2	58.3	58.9	60.2	60.8	62.1

Your ring should be comfortable enough to fit your finger and so that it will not fall off. You should allow enough room for the ring to slide over your knuckle

Your finger changes size during the day depending on the time and the weather – fingers are usually smaller in the early morning or when they are cold and are larger when they are hot or have been used a lot

While the above method is a good guide, if you are still unsure you can visit your local jeweller and they can measure your finger/s for you.