How bra sizes work as of 2012-11-18 17:30:07

A bra size has a number and a letter. The number indicates the length of the band of a bra, the letter is chosen by the difference between the overbreast measurement and the band measurement. The farther along a letter in the alphabet, the bigger the cup.
That means that the band measurement and the cup measurement are independent of each other and therefore they (should) cater to women with all combinations of back measurements and breast sizes.

Band size

The band size is determined by a number which is related to the circumference of your torso.

Early bras were produced before elastic material and, for some reason, the old measurement tables are still being used, even though nowadays bra bands are elastic and therefore stretchy. This easily explains why the +4/+5-sizing method doesn’t work very well if you want your bra band to support you.

Cup size

The cup size is determined by a letter, which is chosen by the difference between the overbreast measurement and the band measurement.

A cup size always depends on the band length! You can't say "I wear a D cup" because a 34D and a 42D are totally different sizes! A person who wears a bra in the size 34D has a much smaller breast circumference than a person who wears a bra in the size 42D. Only if you see cup size relatively to the band size, they have the same difference between their breast circumference and their band length.

A lot of us find that in order to have a bra band that supports you, you have to choose a band length that sounds much shorter than "usual" lengths, cup sizes have to change as well. If you used to wear a bra in the size 34C and find that a band length of 30 fits you much better, you would have to wear a 30DD to have the same cup size as before. See sister sizes to find out why. This explains why a lot of the companies which are favored on Bratabase only start to produce bras at a D cup size.

Sister sizes

Two sizes are considered sister sizes when their cups are the same size, but the band length changes. Usually, a sister size is found when adding or subtracting one cup when going up or down one band size.
This can be explained when you consider that a cup size is always determined by a difference between band and overbreast measurement. That
For example: A bra should have cups the same size in 34C, 32D and 30DD. This method isn't entirely foolproof, it may only work for some of us (some women have to add or subtract two cups when changing the band size), but it can be useful when you are between band sizes or a bra's cups fit you well and you don't mind changing the band to accommodate you.

Different sizing systems

There are a lot of different bra sizing systems, some which can be converted to other systems and some which can’t. They can be applied consistently or vary between brands or even models of one brand!
Check Bra Fitting Cidade for an overview of the most common sizing systems.

Size range available

The actual size range available might be much bigger than you imagine. On Bratabase, bras in sizes from 26D to 50N have been added and this size spectrum is ever expanding!

Calculators

There are a number of different bra size calculators on the web. The bra calculator on Sophisticated Pair seems to be the best around. It can give you an idea of where to start when you want to try out a new size.

Resources

On Wikipedia: Bra measurements for a historical overview, bra size standards and further information.