it might help if women understood how bra sizes work, and i say this in every thread about boobies, but here goes.
if you go up in the back, you come down in the cup. and vice versa. I remember when i was told this at Rigby and peller where I and i was FLOORED!!!!
so
if you are wearing, say a 36c from (shudder) m + S, and you think hmm, it is too big round my chest, but ok with my boob. You would try a 34 (inch) and to compensate you would go 'up' one size in the cup to a 'd'.
Also, start your bra on the first hook Loosest), then as it stretches over time you can tighten it.
PLEASE do not add inches, like 5 or so so your under bust measurement. It doesn't work. In the old days , yes thats' how you would do it. Becuase there were not the variety of sizes. If you measure 35 under your breast you are a 36, stick with this then start on the cup sizes.
36 c is the equivalent to 34d, so if you tried on a 36 and thought hmm it's still a bit loose, go down to the 34 but UP in the cup.