Today I landed on Linda's bra calculator. I must say that they are doing something very smart by acknowledging that different manufacturers run different cup scales, so they provide alternative cups for certain cup ranges 
Linda's store offers bras starting from 30 bands, so their calculator won't go below that, but their wide cup range goes from AA to N cups! 
The calculator's algorithm is also written in JavaScript, which means you can look at the code , so I dug a bit further and studied their method. Here is a table that will help you figure out what the calculator is doing behind scenes.
The calculator works using inches and it uses your over bust measurement and ribcage measurement. In both cases it indicates that you wear a bra for measuring.
This is the math bit
Bratabase does not endorse or has any attribution regarding this method or its results, it's been extracted from Linda's bra calculator.
Let X be the difference between your bust and ribcage measurement, we'll use that number here in the table. Write it down so you can look it up
In this table you'll be able to see the band size the calculator will throw. Find your ribcage range in the first column and see for which condition X fits (if any) and it will instruct you how to know your band size. For your cup size, it will give you instructions on what to do with X in regards of the cup number.
| Ribcage range | Band calculation | Cup calculation | ||
|---|---|---|---|---|
| Less than 29" | Is X less than 2.5"? | Is X greater or equal than 2.5"? | Is X less than 2.5"? | Is X greater or equal than 2.5"? |
| Add 2 to your ribcage and round to the immediate superior even number. That's your band size | Your band size is 30 | Cup number is X | Cup number is X - 1 | |
| 29" up to less than 40" | Is X less than 5"? | Is X greater or equal than 5"? | You cup number is X | |
| Add 2 to your ribcage and round to the immediate superior even number. That's your band size | Round your ribcage to the immediate superior even number. That's your band size | |||
| 40" or more | Round your ribcage to the immediate superior even number. That's your band size | |||
At this point you know your "cup number", look it up in this table to find your possible cups. This is what's good about this calculator, it knows that not just one cup will be best for a bust difference to it suggests two. And see how it uses half inch ranges instead of full inch, so it can show you in between cups .
| Cup number is between | Cup is |
|---|---|
| 0 and 1 | AA |
| 1 and 1.5 | A |
| 1.5 and 2.5 | B |
| 2.5 and 3.5 | C |
| 3.5 and 4.5 | D |
| 4.5 and 5.5 | DD or E |
| 5.5 and 6 | DDD or F |
| 6 and 6.5 | F or FF |
| 6.5 and 7 | FF or GG * |
| 7 even | G |
| 7 and 7.5 | G or GG |
| 7.5 and 8 | GG or H |
| 8 even | H |
| 8 and 8.5 | H or HH |
| 8.5 and 9 | HH or I |
| 9 and 9.5 | I or J |
| 9.5 and 10 | J or JJ |
| 10 and 10.5 | JJ or K |
| 10.5 and 11 | K |
| 11 and 12 | L |
| 12 and 13 | M |
| 13 and 14 | N |
Note that all the intervals are inclusive on left and exclusive on right.
What I still find a bit strange is the complex conditioning regarding the +2 for band size in some cases. The unexpected results are:
| Ribcage | Bust | Bra size |
|---|---|---|
| 31 | 35.5 | 34DD/E |
| 31 | 36 | 32DD/E |
See how by increasing the bust line from 35.5" to 36" results in a smaller band. Weird since the ribcage measurement never changed.
* I think that's a bug in Linda's calculator, I think they meant to say FF or G
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