If you have a ring that fits, have your jeweler check its size for you. Otherwise have your jeweler fit you for a ring. The correct size is determined by how the ring feels once it is on. It is normal to have some tightness getting a ring over the knuckle, even moderate difficulty. Once it is on, however, you should have no sensation of pressure. If you are buying a wide ring, ask your jeweler to use wide-size ring sizers. Usually you have to add half a size to measurements made with a normal-size ring sizer.
Methods such as using a string, or tape measure, or strip of paper are not accurate.

If you are buying a ring for somebody and want it to be a surprise and don't know the size, try this: Get one of the rings that they wear and slide it as far as it will go on any one of your fingers (it doesn't have to go all the way). Draw a line on your finger with a ballpoint pen on the wrist side of the ring. Go to a jeweler and see which size of one of his ring-sizer rings fits up to the line in the same place. That's the size to order.

If you don't want to go to a jeweler, we have a set of plastic ring sizers available for $6.00 including postage (USA). Click to see. These are plastic rings in a graduated set with the ring sizes marked on them. They are very accurate. It will take about a week to get a set to you.

In judging if a size is right, here are some suggestions: It is normal for a ring to feel snug, even tight, going over the knuckle. It is how it feels once it is on that counts. Is there any sensation of pressure? Do you feel aware of it? If so, the size you should order should probably should be a quarter or half-size larger. Remember: in warm weather fingers tend to swell a bit. The opposite in cold weather. And if your hands are wet, a ring can slip off. Try wetting your hands and see if the ring-sizer ring slips off too easily.