Best: To find your ring size, we recommend swinging by your local jewelry store for the most accurate sizing. If you are buying a ring as a gift, you can easily borrow a ring and get it sized.
Good: You can use a very inexpensive plastic ring sizer available on Amazon here: Plastic Ring Sizer
Fair: Many printable ring sizers are available online, however these are much less accurate.
Your ring should fit snuggly on your finger without risk of falling off. Not too tight. Not too loose. It should just be able to fit over the largest part of your finger, which the knuckle for most people.
It is really important to check your ring size more than once whether checking at home or sizing in-store with a jeweler. Your measurements typically change due to the weather and time of day. Check your size on a cold day and again on a warmer day; in the day and in the evening.