There are a couple of methods when trying to get a ring size without the recipient being present at a jeweler's. Russell Kwiat, brand manager of Kwiat jewelers, suggests borrowing a ring from the recipient’s jewelry box to “trace the inside of the ring on a piece of paper or make an impression of it in a bar of soap.” (Reference 1) While it may not be the most precise measurement, it serves as an approximation. To determine your exact ring size, measure the finger the ring is intended for, rather than the impression or outline of a comfortable ring.
Wrap a piece of string around the selected finger. Place it near the base of the finger below the knuckle. Pull the string gently to remove any slack.
Mark the string with a marker or pen where the two sides of the string touch. Remove the string and wrap the finger again to double-check your mark.
Remove the string from the finger and snip the string.
Lay the string flat against a metric ruler. Note the measurement in millimeters for a precise fit.
Compare the measurements to a ring-sizing chart, or convey the information to a jeweler.
Substitute a thin strip of paper in the absence of string. For U.S. sizing, whole sizes vary by a 1/10th inch or approximately 2.5 millimeter increments, but half sizes vary mathematically. Consult a chart.