UPDATE* (2011/06/22), Playtex decided to remove their calculator until resolving the issue. A total Win for Busts4Justice.
They've done it again!, Busts4Justice is now looking at Playtex' bra calculator .
Their technique is pretty standard, measure on the fullest part of your bust, then right underneath your breasts. Enter both numbers and voila~ Bra size!
I took a closer look at how they do it, since they do the calculations on your browser, that means you can take a peek at the code and see the math
Even though you enter inches in the sliders, the math behind works with CM, that's why you get those funny fractions and hardly ever round numbers, the bust can range from 77 to 148 cm (30.3", 58.3".
Then they ask for your ribcage measurement, this range has been delimited by your previously entered bust number. So your choices are limited as well. They have some confusing logic to allow you to enter only valid numbers and never go out of range.
With those numbers, you get the desired bra size. (Although with some bugs, for instance if you enter the least bust measurement, no matter what ribcage number you provide, your bra size will always be 30A.)
The ranges they calculate also make sure that the measurements difference always results in a cup from the cups array determined in the code which goes from A to FF.
I took the liberty to test their code and came up with this:
You can link directly to it if you want from here. Keep in mind that that calculator does not limit the numbers you enter, so you can easily fall "Out of range".
It turns out, they add 13cm to your band size, those 13cm represent 5.1" that are added to your ribcage measurement.