A good few years ago now, after losing some weight I treated myself by going into a specialist lingerie shop. I'd never dreamt of going somewhere like that before- everyone I knew just bought their bras from M&S and guessed at the size by trial and error.
This lovely shop was well staffed with helpful assistants who offered a measuring and fitting service. I'd never heard of this before, but they were so matter-of-fact about it, there was no embarrassment.

A tape measure was used, but mostly estimation of size was done by an experienced eye. The assistant fetched a few bras to try on, and then "fine-tuned" the fitting from how they looked on. It wasn't just the written size, but also the make, as she knew from experience that certain makes are more suitable for certain shapes.

I'd gone into the shop wearing a 40C and I came away with three bras (expensive but worth every penny) size 34E, far better fitting and giving a nicer shape.

It is said that most women are going round in the wrong size bra which doesn't surprise me, as your size changes with adding/losing weight, pregnancy, hormone changes etc.
I have since taken my daughter and daughter-in-law to the same shop, and it's about time I went again.

I'd definitely recommend every woman to get a proper fitting such as I had, every once in a while. It does wonders for your self-esteem to know that everything is being supported and presented in the best way possible :)