First of all: the band goes under the breasts, so why is he measuring her above the breasts ? This doesn´t make any sense.
I try his measuring system out on me:
I am 27" under the bust, and I wear band size 28. Measuring above the breasts would give me 30", and that would mean I would have to go up a band size if I took these measurements seriously ! 28 bands give me support without being too tights. 30 bands would be ok only when they are brand new, after the first wash I could probably wear them around my hips. I do wear 30 bands only from brands like Discover Mademoiselle and Ewa Michalak which make them really tight.
Now to the second part. Lets say I accepted the band size 30 for a moment. My breast measurement is 33". According to him, 3" = 3 cups. So he would fit me in a 30C. I wear a 28DD. If I wear a band 30, I need to cup D, because when you go up a band size you need to go down a cup size (so says the band-cup relativity law). 28DD translates to 30D, not just in theory but in practice: a 30C would be too small in the cups for me.
His measuring system wouldn´t work even if I subtracted my bust measurement from my underbust measurement (instead of overbust). 33" bust - 27" underbust = 6". That would make me a US DDD cup. That´s one size bigger than my correct size.
Where did he go wrong ?
In the UK measuring system, 1" difference between underbust and breast size does stand for one cup size. With one exception: the A cup is a 2,5" inch difference !
So it goes like this -- 2,5"=A, 3,5"=B, 3,5" = C.
If the difference is less then 2,5", you get AA cup (1,5") and AAA (1" or less -- I think)
So what´s the right was to calculate the bra size ?
The band size should be the number closest to your underbust measurement (In my case 27" translates to UK/US band 28). If you are using EUR band sizing, then take away 5 cm from your underbust measurement, and go down to the nearest band size (In my case 69cm -5 = 64cm, then I drop down to the next band size 60).
For the cup size, subtract bust measurement from underbust measurement. Allot 2,5" for cup A, and then every extra inch is a cup size. (In my case: 33" - 27" gives 6". So I take cup DD even though I lack half an inch, but it´s always better to take a slightly bigger cup than one too small, especially because of breast tissue migration). Of course, this size in only theoretical -- depending on your breast shape and how firm - non-firm it is, you might need to go up or down a cup size.
Or just screw all that and just use the calculator:
Click here for the bra size calculator
Congrats, you just broke out of the Bra Matrix !