1. Simply measure the circumference (distance around) your finger (with something flexible, such as a strip of paper or piece of string, and a ruler with millimeter measurements).
2. Or, measure the diameter (distance across) in millimeters of a ring you currently wear. Find the nearest measurement on the chart below. That's approximately your ring size. (When in doubt between two sizes, it is always safer to go to the next larger ring size. This ensures your new ring will fit you.)
3. Compare your measurement with this chart to determine your ring size.
To get your girlfriends ring size, ask to clean her rings the next time in the mall. Most jewelry stores will do it for free. Then you can secretly ask the sales person to check the size for you. Or you can slip her ring on your finger and mark the spot it fits. Then measure that spot with the directions above.
The inside of the ring in the image above is approx 18mm which makes it a Indian Size 15 in the chart below