I just spent a week staying with friends… and, of course, talked bras. A few people asked me to measure them.
An alleged 32B found out that she is, in fact, a 30DD. A “32DD” turned out to be a 28G (US)/28F (UK). And someone already wearing a 30FF (UK) realized that she might need a 28G (UK). Fun times were had by all.
Now, in case you’re curious about how I size people… I don’t usually use the charts that give cup sizes by the difference between the bust and underbust measurements… because, at least in my case, it’s very wrong. Instead, I start by measuring the underbust. I use that measurement as the band size (possibly adding an inch) and then look at the bra that they are wearing. If, for example, your underbust is 28 inches and you are wearing a 32DD (and those cups are the right volume), you should try a 28G (US)/28F (UK). If the cup is too large or too small, move in that direction from the 28G. Of course, different bras fit differently, so there is no exact science here… but this method seems to work best for me.