OK, please ignore the M&S (and most other online calculators) as they are completely useless.
Measure under your bust (hold the tape tight) to get your band measurement. Do not add on any inches to this measurement. Then measure across the fullest part of your bust. Each 1" of difference between the two measurements equals a cup size. So, let?s say you measure 30" underneath and 35" across? That would give you a starting point of 30DD.
Usually you should take the across bust measurement whilst wearing your best fitting bra (preferably an unpadded one although slight padding shouldn?t make too much of a difference). If you think that your bras are fitting so badly that you'll get a distorted measurement, then measure without a bra, but lean forward so that your boobs are hanging towards the floor (attractive!). The cup size is always harder to get right by measuring than the band size.
To test the band size, put the bra (in this example a 30 back) on back to front on the loosest hooks. If you can breathe but if feels tight and you can only fit two fingers underneath the wire at the front, then that's the correct back size for you. If it's so tight it's unbearable then go up a size.
The cup will differ much more between brands and styles than the back. Turn the bra round the correct way, lean forward then take your right hand and put it inside your left cup. Reach right round under your armpit/towards your back and scoop all the backfat/escapage forward in to the cup of the bra. Repeat at the other side.
Then put the shoulder straps up and adjust as necessary (not too tight as it's the back that should be doing most of the supporting work). If for example, you've started with a 30D, you will probably find that after scooping, you have some bulging over the cup and four-boobage going on! This means you need to go up one or more cup sizes. Keep going up through the cup sizes until all your breast tissue is firmly in place.
A good fitting bra?
If your bra is a good fit after doing this it should:
? Be worn on the loosest hook (when new). If you can fasten it on the tightest hook then try a back size lower.
? The band should be completely horizontal
? You should only be able to fit a couple of fingers under the front of the bra. If you can get more than that, or you can pull it more than an inch or so away from your body at the back, it is TOO BIG!
? If you raise your arms, the back should not really move - no riding up
? The wires should be entirely encasing your breast tissue - breast tissue goes surprisingly far back and a lot of people don't realise this. If you feel where the wire is sitting under your arm then it should be sitting on a fairly firm surface.
? The central gore (where the wires meet) should sit flat between your boobs. if it is being pushed away, then the cup is probably too small (and the back possibly too big as well)
? There should be absolutely no lumps over the top of the cup. Take a fitted t shirt with you to put over the top of the bra and stand side on to a mirror and watch as you breathe in. If you have a bit of a bulge only when you breathe in, you probably need to go up 1 cup. If it is visible when breathing out then it's probably more like two cups or more.
? The cup shouldn't gape. If it does, it may be too big or it may be the wrong shape of bra for you. Moulded cups are particularly bad for this as they don't adapt to your shape.
? The shoulder straps shouldn't be painful and digging in. Most of the support should come from the back. If you find that you can't get enough support without the straps being so short they hurt the bra isn't for you. Most likely it's too big in the back but it may also just be a bra that doesn't suit you.
? and on that note; not every brand works for every person. There are quite distinct differences in shape between brands which mean a particular brand may just not suit the shape of your breasts.
(With thanks to StatiscallyChallenged and HazelNutt)