It is important to make sure that your ring will fits you perfectly.
Remember that due to temperature changes your finger may shrink in the cold or expand in the heat. The best time to measure your finger is at the end of the day and when your fingers are at room temperature.
Your ring should fit comfortably, so that it is tight enough not to fall off, but loose enough to slide over your knuckles.
In case you are not sure enough, we recommend you to visit a local jeweler and got sized in person.