Wear a non-padded bra or no bra
Use a soft measuring tape
Measure snugly around the ribcage directly under the bust
Round the underbust measurement to the nearest whole number
Measure around the fullest part of the bust
Keep the tape level and not too tight
Round the bust measurement to the nearest whole number
Subtract the underbust measurement from the bust measurement
Use the difference to determine cup size
Use the underbust measurement to determine band size
Try on bras in the calculated size
Adjust size if the band feels too tight or too loose
Adjust cup size if there is gaping or spilling
Re-measure regularly since body size can change