Wrap a strip of paper or string around the base of your finger
Mark the point where the paper or string overlaps
Lay the paper or string flat against a ruler
Measure the length in millimeters or inches
Use that measurement to find your ring size on a sizing chart
Measure your finger at the end of the day for the best fit
Measure the same finger more than once to confirm accuracy
Avoid measuring when your hands are cold or swollen