Paper strip: wrap a thin strip around the base of the finger, mark the overlap, measure in mm — that is the circumference. Ring size (ISO) = circumference in mm.
Existing ring: measure the inside diameter in mm and compare with our ring size chart.
Printable sizer — check it prints at 100% scale.
Tips
Measure at the end of the day when fingers are largest.
Wide bands (6 mm+) fit tighter — go up a quarter to half size.
If your knuckle is large, choose a size between knuckle and base.