Wear a non-padded bra
Use a soft measuring tape
Measure snugly around your ribcage under your bust
Round to the nearest whole number
Add 4 inches if the number is even
Add 5 inches if the number is odd
This is your band size
Measure around the fullest part of your bust
Round to the nearest whole number
Subtract your band size from your bust measurement
1 inch difference = A cup
2 inch difference = B cup
3 inch difference = C cup
4 inch difference = D cup
5 inch difference = DD or E cup
6 inch difference = DDD or F cup
Combine your band size and cup size
Try the calculated size on
Check that the band sits level
Check that the cups fully contain the breasts
Check that the straps do not dig in
Adjust size if the fit feels uncomfortable