If you don't like the fit of your current bras, pull out a tape measure and let's figure out what size you are in 4 easy steps
Wear your best-fitting, unpadded, underwire bra and nothing else. Grab a soft tape measure that is in inches.
Wrap a tape measure around your ribcage, following the base of your bra band - pull taut so there's no dip in the tape, but not so tight that you create an indent. This number, in inches, is your band measurement. If the number is odd, round up to the nearest even.
Move the tape around the fullest part of your bust, typically at the center of your nips, wrap evenly around your back, holding taut without any dipping. Take this measurement in inches, this is your bust measurement.
Take your bust measurement (second one) and subtract your band measurement (first one). Each inch difference between your band and bust measurements equals one cup size. The band measurement and cup size combined are your underwire bra size.