Wear a non-padded bra or no bra
Use a soft measuring tape
Measure snugly around your ribcage directly under your bust
Round that number 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
Keep the tape level and not too tight
Subtract the band size from the bust measurement
Use the difference to find your cup size
1 inch = A cup
2 inches = B cup
3 inches = C cup
4 inches = D cup
5 inches = DD or E cup
6 inches = DDD or F cup
Try on bras in the calculated size
Adjust one band size up or down if needed
Adjust one cup size up or down if needed
Check that the band feels snug and stays level
Check that the cups fully contain your breasts without spilling or gaping
Check that the straps do not dig in or slip off
Re-measure if your body changes or the fit feels off