Wear a non-padded bra or no bra
Measure your band size around your ribcage, just under your bust
Keep the measuring tape snug and level
Round the band measurement to the nearest whole number
Measure your bust around the fullest part of your breasts
Keep the tape level and not too 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 bras in your calculated size to confirm fit
Adjust for different brands and bra styles if needed