It is important to measure your size carefully and precisely. Refer below for a sizing chart which shows US and UK/AUS sizing. The most common size for ladies is a 7 (N, O, P) and for men is a 10 (T, U, V). The best way to work out your ring size is to measure the inner diameter of one of your own rings and then refer to the chart below. 
Another way is by wrapping a 5mm wide strip of paper around your finger and sticking the ends together to form a ring – ensure you allow a ring large enough to fit over your knuckle. Take off the piece of paper and measure the inner diameter. If you are in between sizes go for the larger size.