Wear a non-padded bra or no bra
Measure your band size around your ribcage, directly under your bust
Keep the measuring tape snug and level
Round the measurement to the nearest whole number
Measure your bust size around the fullest part of your breasts
Keep the tape level and not too tight
Subtract the band measurement from the bust measurement
Use the difference to determine your cup size
Try on a few sizes to check the best fit
Recheck your measurements regularly