Finding your finger size / ring size. : eBay Guides

To find your ring size cut a strip of paper approximately 1cm wide and wrap this around your finger. Mark the paper where it meets so as to work out the circumference of your finger. Measure the marked length on the piece of paper. This is your metric size. It is recommend that you take your ring size towards the end of the day and when your hands are warm (room temperature).

Alternatively you could visit a jewellers or a jewellery dept. in a store and ask them to measure your finger.