Wear a non-padded bra or no bra
Measure snugly around your ribcage, directly under your bust
Round the underbust measurement to the nearest whole number
If the number is even, add 4 inches
If the number is odd, add 5 inches
Measure around the fullest part of your bust
Round the bust measurement to the nearest whole number
Subtract the underbust number from the bust number
Use the difference to find your cup size
Combine the band size and cup size to get your bra measurement