[wee2er]

Actually there seems to be a standard for all kinds of clothing, inclusive bras: ISO-8559 series. I found some draft versions for free (The released ones are costly!).

However, after some informal research I found a rule-of-thumb way of finding the starting point for actual fitting.
1. Measure the circumference "band size" under bust and "bust size" over the boobs.
2. Round the band size in Metric system downward to nearest 5cm, or in Imperial system downward to nearest even number. That is the Band Size number
3. Calculate "Bust Size" - "Band Size" difference. The result is then rounded downward to nearest 5cm / 2 inches. Calculated and rounded difference 0 would be AA, 5 cm/2" A, 10 cm/4" is B etc. The size system for the result is EU/International.

That method should work reasonably well according to my very limited studies, and give a meaningful indication from which sizes to start fitting. I guess the needs caused of varying shape and support requirements are increasing with breast size, so quite possibly for about D cup and bigger only the band size indication is close to final fit. I have not had an opportunity to verify that calculation method with big breasts because I still carry quite small ones, according to the algorithm to be "B size"

OMG, knew I should have searched ISO before commenting..... many thanks for being a proper engineer and doing the research first. good ole ISO 8559, a bit of bed time reading LOL