Below is a sizing guide using US and UK ring sizes:
SMALL = USA 5, UK J/K - diameter 15.7mm
SMALL/MED = USA 6, UK L/M - diameter 16.5mm
MEDIUM = USA 7, UK N/O - diameter 17.3mm
MED/LG = USA 8, UK P/Q - diameter 18.1mm
LARGE = USA 9, UK R/S - diameter 19mm