Here are some things to keep in mind when you are sizing a ring yourself. Remember, nothing is as accurate as having a jeweler complete a ring size for you, but there are tips to making it work better if choosing to size your own finger.
If you measure your finger and find out that your size is in between two different sizes, you should pick the larger size for the ring. Remember that due to temperature changes, your finger may shrink or expand. In the cold, fingers tend to shrink, causing a smaller measurement. In the heat, your fingers swell, which can make for a larger measurement. Take the temperature into account when sizing your finger.