Beware of any website, store, brand, or size calculator that uses "American bra sizing"! There is no such thing as a standard of sizing in America. Most brands that market to America and reputable stores in America will usually use UK sizes. That is, in proper UK sizing, the cup sizes are AA, A, B, C, D, DD, E, F, FF, G, GG, H, HH, J, JJ, K, KK, L. Each size is one size bigger than the one before it--there is no such thing as "half sizes", some of the cup sizes are just called by a double letter while others are called by a single letter. One really odd thing I have seen is that certain online retailers only stock the single-letter sizes, as though they believe that the double letters are "half sizes". Be cautious with retailers like that. If you see anyone listing sizes in a way different than what I've listed above, be careful when ordering from them because what you get may not be what you expect. If you encounter an American bra whose cup size is listed as "DDD", this is theoretically the equivalent of a UK size E, but often fits more like a UK DD. It's not that a DD is equal to an E in theory--UK sizing contains a DD and an E--but more that US D cups seem to often run smaller than UK D cups. Check out this article on the Butterfly Collection blog for more information on this massively confusing gray area in sizing. In general, brands that use DDD do not go above DDD, or do not know how to use standard sizes above DDD, so it's a red flag.
Also, beware of any site, company, fitter, or friend who says that you should add 4 or more inches to your measurement to get your band size! It really depresses me that after years of blogger activism we STILL need to be talking about this, but a lot of people are dead wrong and set in their ways, so you have to have the facts to protect yourself. Ever notice that people who say you should add 4"-6" to your underbust measurement to get your band size never say WHY you should add inches? Yet it is easy to explain why you should NOT add inches--the band is meant to provide most of the support for your boobs. So if it is not firm, the weight of your boobs will pull it up your back, so it's no longer horizontal halfway between your shoulders and elbows. When your band is too big and rides up, your bra is useless and can't give any support, so you'll be at risk for shoulder and back pain, especially if you don't naturally have the best posture (and, SURPRISE! Having heavy boobs and an unsupportive bra tends to wreck your posture just a bit). Adding 4-6 inches is a recipe for misery. It is an outdated convention, back from the days when bra bands used to be made of non-stretch material, so you needed to add inches so you could breathe out. Now, bras are made of stretchy material--so those who say to add so many inches are basically saying you should wear a bra that is already 2-3 sizes too big, AND has a lot of stretch! No one would do this with ANY other article of clothing--imagine how stupid we'd think people were if they did! ("My measurements put me in a size 8, but I added five inches to everything so now I'm wearing a size 12. I wonder why it is so ill-fitting??") If anyone tells you to add 4-6 inches, tell them what I've written here while backing away slowly. Those people shouldn't be coming near your boobs.
I do feel that it is SOMETIMES okay to add no more than 2 inches to your underbust measurement to get your band size. Some people may find they are more comfortable in a band that does not exactly match their measurements, especially those who have ribs that stick out (which can happen at any size). Use riding up as a way to test if your band size is okay. If you are wearing one band size bigger than your measurement, and your band rides up, you will probably be more comfortable in the smaller band size. But if it's not riding up, it sits horizontally, you are comfortable, the center gore sits flat on your chest, and it doesn't give you a backache, then your band size is totally fine. No one has the right to bully you into adding inches to your band size, but you are informed, you will be able to make the best choices for your own body.