Wrap a strip of paper or a soft measuring tape around the base of the finger
Mark where the ends meet
Measure the length in millimeters
Compare the measurement to a ring size chart
Measure the finger at the end of the day when it is largest
Measure the exact finger the ring will be worn on
Repeat the measurement 2 to 3 times for accuracy
If the knuckle is larger than the base, measure both and choose a size that fits over the knuckle
If between sizes, choose the larger size
For the most accurate result, have a jeweler measure your finger with a ring sizer