Wear a non-padded bra or no bra
Measure snugly around your ribcage, directly under your bust
Round to the nearest whole number
Measure around the fullest part of your bust
Keep the tape level and not too tight
Round 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 and adjust if needed