Wear a non-padded bra or no bra
Measure your underbust snugly around your ribcage, directly under your breasts
Round the underbust 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 underbust measurement from the bust measurement
Use the difference to determine 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 the calculated size
Adjust band and cup size as needed for fit
Check that the band sits level and the cups fully contain the breasts