MEASURING YOUR RING SIZE
Measure the INSIDE diameter (from inside edge to inside edge at the widest part) of an existing ring that fits you.
The best way to get an accurate size is to go in to any jewelry store and ask them for your ring size (it's free).
Each finger can be a different size though, so make sure you measure the finger you'll be wearing it on!
BUYING THE RING FOR SOMEONE ELSE
Ask the person's partner/family if they know the person's ring size, or borrow one of their favorite rings and use the 'Measuring Your Ring" technique (above) to determine the size for yourself.
Your ring should fit your finger comfortably; snug enough so that it will not fall off, but loose enough to slide over your knuckles without discomfort. Your finger size can change depending on the time of day, and the weather!
All the above ring sizes are in US format.