Wear a non-padded bra or no bra
Measure your underbust snugly around your ribcage, just below your breasts
Measure your bust around the fullest part of your breasts
Subtract the underbust measurement from the bust measurement
Use the difference to estimate 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
Check your band size using your underbust measurement
Try on a few sizes to confirm fit
Make sure the bra lies flat against your ribcage
Make sure the cups fully contain your breasts without spillage
Make sure the band feels snug but comfortable