The easiest and most accurate way to find out your ring size is to go to a jeweller and have your ring size measured, alternatively go to http://findmyringsize.com if using this method please be very sure of which size you choose, and do it a few times.
Tips for gift buying: To ascertain the appropriate ring size, ask to clean a ring of the gift recipient and take the ring to a jewellery store. Most jewellery stores will perform this service free of charge. Then ask the jeweller to check the size of the ring or alternatively you can slip the ring on your finger and mark the spot it fits. Then measure that spot using the dimensions below.