If you have large and / or sagging breasts then when the person is taking your band measurement, you should pull up on your bra's shoulder straps so that she can get the measuring tape under your breast for an accurate measurement. (thefashionablehousewife.com)
For example, a 3 inch difference between the band measurement and the bust measurement would indicate a C cup size, a 4 inch difference would indicate a D, etc.. (motherandchildhealth.com)
The difference between the band measurement and the cup measurement in inches determines the cup size. (motherandchildhealth.com)