Determine proper bra size by first measuring the band size, then determining the bust size. From these measurements, find the proper cup size and final bra size.
First, wearing no bra or a non-padded bra, wrap a fabric measuring tape snugly around the chest, making sure to keep it level all the way around. Place the tape directly under where the bust meets the chest, and measure. Add four to this measurement if the measurement is even and five if the measurement is odd. This new number serves as the band size.
Next, wrap the measuring tape a little loosely around the bust at nipple level, again keeping the measuring tape level. Round the measurement to the nearest whole number. This number serves as the bust size.
Finally, subtract the bust size from the band size. The difference between these determines the cup size. If the difference is 0, the cup size is AA; if 1, then it is A. Each progressive whole number corresponds to the next cup size through B, C, D, DD, DDD/F and G.
The final bra size is the combination of the band size and cup size: for instance, 34 (band size) C (cup size). The bra size can be checked by ensuring that the back of the bra sits level with the front, putting on a tight shirt and looking for puckering or bulging, and making sure that your breasts in the new bra size sit midway between your shoulders and your elbows. If the bra size has to be adjusted based on these determinations, change cup and bust sizes inversely (up a cup size, down a band size, and vice versa).