Wear a non-padded bra
Measure your underbust snugly around the ribcage
Measure your bust at the fullest part of the chest
Subtract the underbust measurement from the bust measurement
Use the difference to find the 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
8 inches difference = H cup
Check sizing charts for brand-specific variations