Head in to your local Grahams store and have them measure it for you. All of our stores are equipped to help you determine your ring size. Ensure that the jeweller uses a ring sizer of similar width to the ring you are interested in. For example if you are looking for a 6mm wide ring and you have your finger sized with a 3mm wide sizer you may get the false impression of comfort and then when you buy the ring it will feel a bit tight. So if you are buying a narrow ring have your finger size measured with a narrow sizer, if you are buying a ring of over 3mm width, have your finger size measured with a wide sizer.
The quickest way to determine your ring size is to use the Ring Size Tool located on this page. You will need to have access to an existing ring that already fits and a credit card to determine the appropriate size.