I've said this before on the subject of bra sizing, but I'm gonna repeat it again, sorry!
The 'standard' starting point is to measure round the ribs, under the breasts and then add 4 or 5 to get an even number and that is your band size. Cups are 1 size increase for every inch your bust size ( round the nipples) is greater than the rib measurement.
The two big problems with that are:
(a) That they were derived back before WW2 by one of the large manufacturers of the time ( possibly Berlei but I can't quite remember) and female body shapes have changed markedly since then, both in bone structure, body fat levels and simply physical size because of better nutrition. It also takes no account of physical differences due to racial origin. Finally, of course, from our POV it makes no allowance for the differences between male body shape and female body shape, on top of all the other changes.
(b) When the system was devised, fabrics were extremely limited compared to now, and straps etc were all non-stretch apart from possibly a small piece of wide elastic at the back fastening, and some bras didn't have that even in the 50's.
Because of that, the "plus 4 or 5" was necessary to allow breathing and make use of body fat to give some comfort ( put a snug belt round your ribs and see what its like to try to do anything - unless you like a tight laced corset
).
Now there are loads of stretch fabrics available so the tightness isn't an issue, there are now countless manufacturers and all have their own sizing methods, and there are the body shape variables I mentioned above. The bottom line is that the actual numbers on bra sizing mean almost nothing, except to get you into roughly the right area.
If you buy from one manufacturer then you might find your size and stick to it, but there will be differences from one make to another.
If you buy own-brand from a big chain store/supermarket, then you will almost certainly find they get them from an assortment of far-eastern manufacturers and there is little consistency from model to model even within one brand.
My ribs measurement is 35". In 'theory' that means I should wear a size 40. I currently wear a 36 and that is beautifully comfortable, a 40 would fall off and do nothing. Also, the edges of the underwire, at the sides, go just to the back of my boobs under the front edges of my armpits and I wouldn't want them to go back any further because it would get uncomfortable.