First, measure your underband size
While braless or wearing a non-padded bra, measure around the underband, directly under your bust. The measuring tape should be level, measure in inches and be very snug. If this number is even, this is your underband size. If the number is odd, add one inch to this number, and then this is your underband size. (So if you measured 32 inches, you underband size is 32. If you measured 33 inches, your band size is 34.)
Now, the cup size
Wrap the measuring tape somewhat loosely around the fullest part of your bust (at nipple level) and round to the nearest whole number.
Subtract your underband size away from your bust measurement and the difference between these measurements will reveal your cup size. Refer to chart below. For example, if your bust measurement is 36 inches and underband measurement is 32 inches, then the difference it 4 inches. Your bra size would be 32D.
Cup Size Chart
MAAREE Conversion Chart
Dress Size Conversion Chart
Bust Conversion Chart
Cup Conversion Chart