Measure your band size.

Run a tape measure all the way around your body just underneath your breasts and take a measurement in inches. Make sure the tape measure is horizontal and fairly snug. Your arms should be down. If this measurement is an odd number, round up to the nearest even number. This should be your band size.[4] If your measurement is already an even number, you may find that this is your band size, or you may have to go up to the next size (i.e, you may have to add 2 inches.) For instance, if you measured 31 inches, your band size should be 32. If you measured 34 inches, your band size may be 34 or 36. Many bra fitting guides and calculators will tell you to add four or five inches to your underbust measurement, but this is not correct. The old method was devised by Warners in the 1930s when bra design was in its infancy and does not work with modern, elasticated bras.[5] Determine your cup size. The most accurate way to determine your cup size is by using your current bra size as a starting point. The cups are sized relative to the band, so if you were to try a smaller band size but keep the same cup size, the cups would be too small. Instead, you must increase the cups by one size for every band that you go down. For example, if you are currently wearing a 34C bra and your underbust measures 31 inches, then you will most likely need a 32D. On a 30" band, this would be a 30DD etc.
In UK sizing, cup sizes are as follows: AA, A, B, C, D, DD, E, F, FF, G, GG, H, HH, J, JJ, K, KK. Try on a bra with the band and cup size you've arrived at in these steps. You should not regard this as your definitive size until you have tried on a few bras, and even then you will often find you need a different size in different brands or styles of bra. Check the band size. The correct band size is the smallest you can comfortably wear.[6] It needs to be tight enough that the bra is still fairly supportive without weighing down heavily on the shoulder straps.
You should be able to run your fingers around the inside of the band, but not much more. A good rule of thumb is that you should be able to fit no more than a fist under the back of the bra. It should fit on the biggest adjustment, but will probably be too tight if you try to fasten it on the smallest size. Bras are designed to fit like this so that you can tighten the band as the elastic starts to wear out. If the band is roomy enough for you to be able to comfortably fasten it on the tightest adjustment, try a smaller band, for example if a 32D is too loose, try a 30DD. Remember that the cup size has to be changed when you move to a different band size - for every band you go down, you must go up by one cup size in order for the cups to remain the same capacity and vice versa. If you can only just fasten the bra and the band is painfully tight, even on the biggest adjustment, then go up a band size, for example if a 32D is too tight, try a 34C. Check the cup size. The correct cup size is the biggest you can completely fill out with no wrinkling of the fabric or space in the cups. You should fill out the cups, but not bulge out anywhere, even in low cut or pushup bras.
Check around the cups for any bulging, not only along the top edges but also at the sides under your arms. Make sure the underwire encloses your whole breast and lies flat against your rib cage. If the cups are too big, go down a cup size. If they are too small, or even if they seem to fit ok, try on a bigger cup size as well to double check. It's a lot easier to tell if they are too big than too small.