Why do John Lewis add on 4 inches to your back size to get your bra size?
After a lifetime in 34As and occasionally 34B an assistant in a small lingerie shop took one look at me and said there was no way I was a 34 back. I am a size 8 usually verging on a 10.
I then began to try on 32 backs and they fitted much better- no always fastening on the tightest hook.
I now wear a 32 C or sometimes a 32D and very occasionally 34C.
HOWEVER just been reading the John Lewis size guide on their website and they are still sticking 4 inches onto your back measurement- so my measurement of 30 inches ( round rib cage) is classed as a 34 inch and they make me a 34B.
So come on you experts- under bust is 30 inches ( just- verging on 29) and over bust is 35. ( or 34 depending how tightly I pull the tape.)