Braology Certified Self Sizing
Most importantly find the correct band size. The band is the entire support structure of the bra. (Most people wear a bra band that is 3-5 sizes too large so prepare for this number to be smaller and fit more snuggly than you're used to.) To find these measurements you should wear your best fitting bra and make sure it has no thick padding in the cup, a light molding will be fine.
Find the band size:
Measure the smallest part of your ribcage right at the base of your breasts, it is imperative that you keep the tape parallel all the way around and pull it tightly. (your bra has more stretch than the measuring tape so pull it very tight). Because by definition a measurement is supposed to be exact, the number of inches you get from this measurement will be your band size. The only exception would be if you would in fact need a smaller band than what you measure. Your measurement can be thrown off and appear too large but it can not be too small, never add inches to the band measurement or you jeopardize all of the support the band can give you. If you measure 33 inches, you would round down to the bra size 32 band. This lengthens the life of your bra by allowing you more time before it stretches out. After you've found a band size that fits securely check to see that when pulled out from the back it only comes away from your back about an inch. if its looser than that you'll need to go down a band size.
Find the cup size:
After you know your band size then you can better establish your cup size. To get a good starting point, measure around the fullest part of your bust loosely and parallel to the ground all the way around your body. Take that number and subtract the band size measurement, for example back measurement is 32" and your bust measures 40" the difference between them is 8". Use your tape measure starting with 1" and say a letter name for each inch up to the number that is the difference, 8 in this example. The letters should be said in the following order. 1=A, 2=B, 3=C, 4=D, 5=DD, 6=DDD, 7=F, 8=G, 9=H, 10=I, 11=J, 12=K, 13=L, 14=M, 15=N, 16=P.... and so on. If your cup size is over an H please know that is very common for most women. The average cup size range is F-K but there are often those on both sides of that range as well... keep in mind its just a letter ladies. The comfort and the shape will be well worth the extra math.