From the manufacturer

Find Your Best Fitting Bra

1) Wrap a tape measure snugly around your body below your bust. After breathing out, take your measurement. If it’s an even number, that is your band size, but if it’s an odd number, round up to the closets even number.

Find Your Best Fitting Bra

2) Wrap the tape measure around your chest at the fullest part of your breast. Subtract your band measurement from this number. If the difference is 1 inch, then you are an 'A' Cup; 2 inch then you are a B Cup; and so on through DDD.

Find Your Best Fitting Bra

3) Combine your band measurement and your cup measurement to determine your correct size.