Wear a non-padded bra or no bra
Measure your band size around your ribcage, directly under your bust
Measure your bust size around the fullest part of your breasts
Subtract the band measurement from the bust measurement
Use the difference to find your cup size
1 inch difference = A cup
2 inches difference = B cup
3 inches difference = C cup
4 inches difference = D cup
5 inches difference = DD or E cup
6 inches difference = DDD or F cup
Try the bra on to check fit
Make sure the band feels snug and level
Make sure the cups fully contain your breasts
Adjust for different bra brands, since sizing can vary