Measure an existing ring’s inner diameter with calipers or a ruler (mm)
Use a ring-size chart to convert inner diameter (mm) to ring size
Measure the finger circumference with a flexible tape measure
Use a ring-size chart to convert circumference to ring size
If you don’t have a tape, wrap a strip of paper/string around the finger snugly
Mark where the paper/string overlaps, then measure the length in mm
Use a ring-size chart to convert that length to ring size
Measure at the time of day your finger is usually largest
Measure twice (or more) for accuracy
Measure the same finger you plan to wear the ring on
If between sizes, choose based on comfort preference (snug vs. loose)