To properly measure bust size, a woman should wear a non-padded bra and wrap a tape measure snugly, but not too tightly, around her from back to front, keeping the back at level and over the fullest part of the breasts in the front.
The bust is only one part of bra size, however. To know the band size, the tape measure must be wrapped around the body at the band level just under the breasts. Subtract the band size from the bust size. Each inch difference is equivalent to a cup size; thus, a 1-inch difference is an A cup, while 2 inches signifies a B.