Wear a well-fitting, non-padded bra (or no bra if you can measure accurately without).
Measure your underbust snugly around your ribcage.
Round the underbust measurement to the nearest whole number (in inches or cm, consistently).
Measure your bust at the fullest part, keeping the tape level and snug but not tight.
Round the bust measurement to the nearest whole number (in inches or cm, consistently).
Calculate the difference: bust minus underbust.
Use the difference to determine cup size based on your sizing system (US/UK/EU vary).
Determine band size from the underbust measurement using your sizing system’s band rules.
If between sizes, choose the band size that feels snug and stays level; then adjust cup size accordingly.
Recheck fit: band should feel firm on the loosest hooks; cups should fully cover without gaps or overflow; straps should not bear all the support.