Measure below your breasts (band size)
Use a soft tape measure. To obtain your band size measurement, measure very snugly around your rib cage, just underneath your breasts. It should feel tight. Be sure that the tape measure lies straight all the way around. This measurement is in centimeters.
After years of experience Rienie has found that most women will take one size bigger in the chest bra than in their normal bra size, providing that you know your correct size, e.g. if you normally use a size 34, your chest bra would be a 36.