If you already have a ring that fits the finger, use its size as reference. Use a ruler to measure the ring's inner diameter: just place the ruler across the ring's centre.

If you don't have a ring, measure the finger:

Cut a strip of paper approximately as wide as the ring you have in mind and wrap it around the finger.

Adjust the fit: the strip should be snug but not too tight. Mark the point where the ends of the strip meet. Unwrap the strip and use a ruler to measure it. The value in mm represents your ring circumference.

Double check: divide the length of your strip by 3,14 — the π number. This is your ring diameter.

If your measurement falls between two sizes on the chart, go with the larger one.