Wear a non-padded bra or no bra
Measure snugly around your ribcage directly under your bust
Round the underbust measurement to the nearest whole number
Measure around the fullest part of your bust
Keep the tape level and not too tight
Subtract the underbust measurement from the bust measurement
Use the difference to determine cup size
1 inch = A
2 inches = B
3 inches = C
4 inches = D
5 inches = DD or E
6 inches = DDD or F
7 inches = G
8 inches = H
9 inches = I
10 inches = J
Check the band size and cup size together for the full bra size
Recheck measurements if the fit feels off