This is my personal soap box subject.
If you go to be fitted for a bra and someone comes at you with a tape measure then run away very fast because it's likely that they're crap, also ignore all the stupid advice to add extra centimetres onto your back measurement.
La Senza, M&S and a few others give that stupid gem out so there are millions of women wearing the wrong sized bra.
At home, measure your chest underneath your boobs, pulling the tape measure tight but make sure you can still breathe.
This is your back size (if it was an odd number then you can try on the numbers above and below to decide which is more comfortable).
Them measure around the fullest part of your boobs, and subtract the number from your back measurement.
For each centimetre different go up a cup size, so a 6cm difference would be a an E cup.
Then start trying on bras until you find the perfect fit.