Wear a non-padded bra or no bra
Measure snugly around the ribcage directly under the bust
Round the underbust measurement to the nearest whole number
Measure around the fullest part of the bust
Keep the measuring tape level and not too tight
Subtract the underbust measurement from the bust measurement
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
7 inches difference = G cup
Check the band size and cup size together
Try on different sizes for the best fit