US sizes run on a numeric scale in quarter- and half-size steps; each full size adds roughly 0.8mm of inside diameter. The chart below covers the range that fits 95% of adults.
Size 4 = 14.9mm · Size 4.5 = 15.3mm · Size 5 = 15.7mm · Size 5.5 = 16.1mm · Size 6 = 16.5mm · Size 6.5 = 16.9mm · Size 7 = 17.3mm · Size 7.5 = 17.7mm · Size 8 = 18.1mm · Size 8.5 = 18.5mm · Size 9 = 19mm · Size 9.5 = 19.4mm · Size 10 = 19.8mm.
Fit tips
Fingers swell in heat and shrink in cold — measure at the end of a normal day, at room temperature. Wide bands (6mm+) wear tighter; consider a quarter-size up. Knuckle noticeably larger than the base? Fit the knuckle, not the base.