Wear a non-padded bra or no bra
Measure your underbust snugly around your ribcage, just below your breasts
Measure your bust around the fullest part of your breasts
Subtract the underbust measurement from the bust measurement
Use the difference to find your cup size on a bra size chart
Check your band size from the underbust measurement
Combine band size and cup size to get your bra size
Try on bras in nearby sizes to confirm fit
Recheck measurements if your size changes over time