Measure the finger at the end of the day
Measure when hands are warm
Use a ring sizer or printable ring chart
Measure the exact finger the ring will go on
Check the size of a ring that already fits well
Make sure the ring can slide over the knuckle
Choose a slightly looser fit for wide bands
Choose a slightly tighter fit for thin bands
Account for swelling, temperature, and time of day
Confirm the size with a jeweler if possible