1. Wrap a thin strip of paper or string around the base of the finger that will wear the ring.
2. Mark on the paper or string where it overlaps on itself.
3. Use a ruler to measure the length of paper or string from its leading edge to the mark.
4. Use that measurement to find the corresponding ring size in the chart.

How To Get The Most Accurate Ring Size:

1. See that the loop of paper or string measuring the finger can slip over the knuckle easily.
2. Measure when the hands are warm. The fingers shrink when they are cold.
3. The hand is largest at the end of the day. That is the best time to measure the finger.

If You Want The Ring To Be A Surprise:

1. See if one of her discreet friends may know her ring size.
2. Borrow, in secret, one of the rings you have seen her wear. Slip it down a tapered candle and mark the place where it stops. Measure around the candle as noted above.