Wear a non-padded, well-fitting bra or no bra
Measure snugly around your ribcage directly under your bust
Round the band 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
Keep the tape level and not too tight
Subtract the band size from the bust measurement
Use the difference to find the 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 band and cup size if the fit is not right
Check that the band sits level and snug
Check that the cups fully contain the breasts
Check that the straps do not dig in or slip off
Re-measure periodically for changes in size