You should be able to fit two fingers under the band comfortably.
A calculated size is just a starting point; different styles like those found at Lane Bryant or la Vie en Rose may fit differently. Use these checks to confirm: how to know what size bra to buy
Example: If your bust is 36" and your band is 34", the 2-inch difference means you are a 34B. Signs of a Perfect Fit You should be able to fit two fingers
Wrap a soft measuring tape around your ribcage, directly under your bust. Keep the tape level and snug against your skin, but not digging in. This is usually your band size. if you measure 31 inches
Round up to the next even number (e.g., if you measure 31 inches, your band size is 32).