Figuring out your ring size is easier than you think. To ensure a proper fit, make sure you measure your finger when you're not too cold or your ring may not end up fitting you. You want your ring to fit snugly over your knuckle. If it comes on too easily, it'll move around on your finger, especially if the stone is larger.
You can either measure a ring that you already have that fits the finger you want the new ring for or make your own ring with a string or wire. (make sure it ends up perfectly round).
Measure the diameter in millimeters (straight across the INSIDE of the ring). Be precise. Now you can look at the chart and see your US ring size. If you're uncertain, go to a local jeweler and have them size you. Most do this as a complimentary service.
If the ring you're interested in is a wider band, it may be wise to go up a half a size as wider bands fit tighter due to their width.