Bra and cup size are obtained in the following way:
Whilst wearing a bra, take the underbust measurement holding the tape measure firmly. Take the measurement in inches (If you measure in centimeters, divide the result by 2.54). If the number of inches is odd, add five, if it is even add four, this result is the bra size. Next take the bust measurement. The difference between the underbust measurement and the bust measurement gives the cup size as follows:
4 inches is an A cup
5 inches is a B cup
6 inches is a C cup
7 inches is a D cup
8 inches is a DD cup

Example:
Underbust measures 32 inches, bust measures 37 inches, waist measures 28 inches, and hips 38 inches.
Underbust is even, so add 4 to give a bra size of 36. The difference between the underbust and bust size is 5 inches, giving cup size B. The correct size of bra or basque is 36B.
Bust size is 37 inches, so for camisoles, nighties etc, size 14 or medium is the correct size.
Waist size is 28 inches, so for briefs, french knickers etc, medium is the correct size.