Measure an existing ring’s inner diameter with a ruler or caliper (mm)
Compare the diameter to a ring size chart (US/UK/JP/EU)
Measure your finger circumference with a flexible tape measure (mm), then use a ring size chart
Use a printable ring-size template, then compare the fit
Wrap string or paper around your finger, mark the overlap, measure the length (mm), then use a ring size chart
Check sizing by trying rings that are close in size and note comfort and movement
Measure at the same time of day and when your hands are warm (avoid cold)
Measure multiple fingers if you plan to wear the ring on a different finger than usual
If between sizes, choose the size that allows slight movement without slipping