Always make sure your hands are warm before measuring your fingers and ensure you measure for comfortable fit - not too tight and not too loose.
Method 1 : Measure the inside diameter in millimetres of a ring that is comfortable on the finger you want to wear your new ring on
Method 2 : Wrap a piece of string or cotton around the base of the finger you wish to size, selecting the snuggest fit that still fits over your knuckle. Make a mark at the point where the string/cotton overlaps. Measure the string/cotton to give you your ring size in mm.
By looking at the chart below, you can use the measurements you have taken to check against various industry standard ring sizes.