The easiest option is to use special probes, which are available in jewelry stores. With their help it is easy enough to determine the exact size, and already on its basis to choose the model.
You can also measure the diameter of the existing ring suitable for you or the person you want to make a gift. For example, if the diameter is equal to 16.5 mm, a size of 16.5.
However, sometimes there are situations when the rings don't have, and the size of your finger to find out. To get out of this situation will help regular thread. Just wrap it around your finger, and then cut in the place where it joins the ends. The resulting length of the circumference of the finger is sufficient to divide it by 3.14 to get the diameter.
If you want to give a ring to another person, you can take the help of friends and acquaintances. They can tell you what size finger ring will work best.
The latter method is suitable only if your fingers are more than the person you are going to do a gift. During the conversation in the form of jokes, try to try on his ring, and then remember how far it got (I can even make a mark). This will allow to know at least approximately the size of a finger ring.