Measure the bust at the fullest part of the breasts with a soft measuring tape
Keep the tape level and snug, but not tight
Measure the underbust area directly beneath the breasts
Subtract the underbust measurement from the bust measurement
Use the difference to estimate the cup size
Use the underbust measurement to estimate the band size
Check the measurements against a bra size chart
Repeat the measurements for accuracy
Measure while wearing a non-padded bra or no bra
Ask a professional bra fitter for confirmation