Measure your ring size at the end of the day when fingers are at their largest. Consider the width of the band, a particularly thick band will feel tighter than a thin band so you may want to buy a full size larger that your normal ring size. Do not measure your hands when they are cold, they may have shrunk up to half a size. Alcohol and salt can make your fingers swell so refrain from measuring your ring size after cocktails and appetizers.