Measure your underbust: wrap a tape measure snugly around your ribcage directly under your breasts
Measure your bust: wrap the tape measure around the fullest part of your breasts, keeping it level
Subtract the underbust measurement from the bust measurement
Use the difference to estimate cup size
Use the bra band size as the underbust measurement rounded to the nearest standard band size
Use the cup letter from the difference between bust and band measurements
Example: 34 band and 37 bust means a 3-inch difference, which is about a C cup
Bra size = band size + cup size
Bust size is the full bust measurement, not the bra size label
If you already know your bra size, the bust measurement is usually the band size plus the cup difference
Check a size chart for the specific brand because sizing can vary