For a ring size conversion chart please click here
Finger sizes change depending on the time of day and the weather.
Some people have success measuring their finger with a piece of string and then measuring the string length. Remember that the string is flexible and metal is not, so while this method is not always 100% accurate, it's a good place to start.
Most jewellers will offer a ring sizing service or can measure your other jewellery.
There are many of printable ring sizers on the web, just remember to make sure that there is a scale to compare to your ruler or tape measure to ensure printing is accurate!