Wear a non-padded bra or no bra
Measure your band size by wrapping a tape measure snugly around your ribcage, just under your bust
Round the band measurement to the nearest whole number
Measure your bust size by wrapping the tape measure around the fullest part of your breasts
Keep the tape level and snug, but not tight
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 on a bra in the calculated size to check the fit
Adjust the size if the band rides up, cups gap, or breasts spill over