Three methods, in order of accuracy: our free plastic sizer (request one — it's the jeweler's tool), measuring a ring that already fits, and the paper-strip wrap.
Method details
Existing ring: measure the inside diameter in mm and match it to the chart — 16.5mm = size 6, 17.3mm = 7, 18.1mm = 8. Paper strip: wrap snugly where the ring will sit, mark the overlap, measure the length in mm and divide by 3.14 for diameter.
Mistakes that skew results: measuring cold hands (fingers shrink a half size), pulling the paper tight (reads small), and measuring the wrong hand — dominant-hand fingers run about a quarter size larger.