Use a ring sizer or ring size chart
Measure your finger at the end of the day
Measure when your fingers are warm
Wrap a strip of paper or string around the base of the finger
Mark where the ends meet
Measure the length with a ruler
Compare the measurement to a ring size chart
Measure the inside diameter of a ring that already fits
Match that diameter to a ring size chart
Make sure the ring can slide over the knuckle comfortably
Measure the exact finger for the hand and finger you will wear the ring on
Repeat the measurement for accuracy
If between sizes, choose the larger size