One way to find your size is to take a ring that you already own and measure it's inside diameter, this measurement you can compare to the chart below to find the letter value.

Otherwise, take a small strip of paper or ribbon and comfortably wrap it around the base of your finger, marking where it overlaps. Flattened out, the measurement of this length gives you the circumference, which you can compare to the chart below.

If there is a large disparity between the size of your middle knuckles and the base of your fingers, it is best to use the first method to ensure you will be able to get the ring on.

Remember fingers are squishy, so if your measurement is within the millimetre of one of the sizes it should still fit nicely.