Wear a non-padded bra or no bra
Measure your band size around your ribcage, directly under your bust
Keep the measuring tape snug and level
Round the band measurement to the nearest whole number
Measure your bust size around the fullest part of your chest
Keep the tape level and not too tight
Subtract the band size from the bust size
Use the difference to determine 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 bras in the calculated size to check fit
Adjust band and cup size if the fit feels too tight, loose, or gappy