The bra band is the entire support structure of the bra. Therefore it needs to fit snuggly to the body. This is why when you measure the smallest part of your ribcage, right at the base of your breasts it is imperative that you keep the tape parallel all the way around and pull it tightly. (your bra has more stretch than the measuring tape so pull it very tight). Because by definition a measurement is supposed to be exact, the number of inches you get from this measurement will be your band size. The only exception would be if you would in fact need a smaller band than what you measure. Your measurement can be thrown off and appear too large but it can not be too small, never add inches to the band measurement or you jeopardize all of the support the band can give you. If you measure 33 inches, you would round down to the bra size 32 band. This lengthens the life of your bra by allowing you more time before it stretches out.
After you've found a band size that fits securely and when pulled out from the back only comes away from your back about one inch then you find a proper cup size that will hold all the breast tissue that no longer fits inside the bra band under your arms. You'll likely go up a few cup sizes if your band measurement is much smaller than you're used to wearing. Remember cup sizes are only letters. The small band size will make your breasts look smaller and more shapely in a much larger cup size. Works every time! You'll look and feel fabulous when your breasts are supported and every part of your body is where it's supposed to be.