This is how to determine your bra size: take a tape measure and measure directly under the chest — that is your band size, then measure the fullest part of the chest. Every inch difference between those two measures is your cup size:
1-inch difference equals an A cup, two inch is a B cup, three inches — a C cup, four inches — a D cup and so on. For example, 40 in chest and 36 in band equals 4-inch difference, that means your size is 36 D.