When I first started buying bras there used to be a formula. Measure under your bust in inches, add 4 if it's even and 5 if it's odd to get an even number, this is your "band size". Measure over the fullest part of the bust in inches, and subtract your band size. If they're the same you're an AA cup, 1 inch bigger = A cup, 2 inches = B cup, 3=C, 4=D, 5=DD for some reason then E and so on. Some of the other letters get doubled too but I forget which.
These days the formula seems to still be in use officially, but in practice you just wear whatever actually fits, and with modern elastic this seems to involve getting a much smaller back size and then adjusting the cup size bigger to compensate.
By the formula I should be wearing a 40A bra (36" underbust, 41" bust - although I measured myself so this may not be perfectly accurate). I'm actually wearing a 36DD. That's quite some difference! But the question then nags me, am I or am I not one of the 80% of women reputed to be wearing the wrong bra size?