1. If you are between sizes, order a larger size. It's much easier to size down on a ring than to size up if you need to have it re-sized.
2. When measuring, make sure your finger is at a normal body temperature. Fingers tend to expand when hot, and shrink when cold, and the difference could be up to a full size or more! So don't measure after you've been to the gym, or if you're outside on a cold day, for example.
3. When measuring, make sure you can slide the ring on and off your finger, over the knuckle. You're looking for a snug, not tight fit.
4. Don't want to order a ring sizer? You can also measure across the diameter (across the inside middle of the ring) on another ring that fits your finger that you're looking to buy the ring for. Use the chart below to find out your size!