Wear a non-padded bra or no bra
Measure your underbust snugly to find your band size
Measure around the fullest part of your bust to find your bust measurement
Subtract the band measurement from the bust measurement
Use the difference to determine your cup size
Try on bras in the calculated size
Check that the band fits firmly and stays level
Check that the cups fully contain the breasts without gaping or spillage
Adjust the straps so they are supportive but not digging in
Recheck size if the bra rides up, pinches, or feels loose