Same as most calculators, you can see the algorithm in the page's source code. So here's how it works:
The website asks you to take 3 measurements:
- Over bust
- Full bust
- Under Bust/ribcage
The hard limits for the calculator are:
- Your over bust measurement can't be smaller than 32 or larger than 46.
- Your ribcage measurement can't be smaller than 25.
The band size is obtained directly from the over bust measurement, if it's an odd number, the immediate higher even number will be used.
The cup size is determined by the difference between the bust and under bust measurement. Once you round down that result use this table to find your cup size:
If your cup is either DD or DDD they will range you in "DD+" section. If the difference is greater than 7 inches or your band smaller than 32, then you'll see this message:
Which I would recommend changing to something else not to make you feel like your boobs are uncalculatable. I think a more appropriate message would be:
And it would rock if they could link to other stores that do offer bigger ranges .
The math is not very difficult on this calculator, as you can see the store stocks sizes from 32AA to 46DDD. Although the "DD+" section does have a limited collection as many of the models shown are available only up to D cup and you can't tell if they carry DD or DDD for each model until you click to see the details.
If their "DD+" range is only DD and DDD, I would not group them in one range and just show them as separate alternatives in their cup selection.