Pop in to your local jeweller and have your finger accurately measured.
If you are not in a hurry a can send you out a "multisizer" ring sizer for you to use at home. If you would like to do this just let us know in the comments field at the checkout page, and we will post one out to you. Once you have worked out your size simply let us know either by phone or email and we will size your new ring and courier it to you the next day. There is no extra charge for this service, it just means your ring will take a few days longer to get to you.
If it is a gift you may be able to ask the recipients friends or family members if they know their size.
Finger size changes during the day. It is better to measure a finger later in the day when it is a bit warmer than first thing in the morning when it is cold as the finger will be smaller.
It pays to check the measurement a few times to make sure you are as accurate as you can be.