It should be level all the way around. If it arches up your back, the band is too big. You should be able to fit two fingers underneath it.
Numbers are just a starting point; the mirror tells the truth. Look for these signs of a good fit: how to buy correct bra size
If the number is even, add 0. If it’s odd, add 1. (Example: 31" becomes a 32 band). 2. Measure Your Cup Size It should be level all the way around
Wrap a measuring tape snugly around your ribcage, directly under your bust. Keep the tape level and parallel to the floor. add 0. If it’s odd