Wrap a strip of paper, string, or a ring sizer around the base of your finger
Mark where the ends meet
Measure the length with a ruler in millimeters
Compare the measurement to a ring size chart
Measure your finger at the end of the day when it is largest
Measure the exact finger you will wear the ring on
Repeat the measurement a few times for accuracy
Use a ring that already fits and measure its inner diameter
Compare the inner diameter to a ring size chart
Check whether the ring size chart uses US, UK, or international sizing
If between sizes, choose the larger size
Have a jeweler professionally size your finger if needed