Wear a non-padded bra or your usual bra
Measure your band (underbust) snugly around your ribcage using a soft measuring tape
Record the underbust measurement in inches or centimeters
Measure your bust at the fullest part, keeping the tape level and snug but not compressing
Record the bust measurement in inches or centimeters
Subtract underbust from bust to get your cup difference
Use the cup difference to determine cup letter based on your measurement system (US/UK/EU)
Confirm band fit by trying the bra: band should feel snug and level, staying in place when you move
If the band rides up or feels loose, re-check the underbust measurement and consider a smaller band size
If the band feels too tight or you cannot breathe comfortably, consider a larger band size
Re-check cup fit: cups should fully cover the breast without gaps or overflow
If there are gaps at the top, consider a smaller cup; if there is overflow/spillage, consider a larger cup
If measurements fall between sizes, choose the closest band size first, then adjust the cup size based on fit