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.

How to Find the Right Sports Bra

Wearing the right sports bra is critical to your comfort, especially during long runs. Follow these steps to make sure you get the perfect one for you.
It's important that you try on sports bras for fit and comfort, so try to do your shopping in person. Go to a running specialty store because they'll have high-impact sports bras that are designed especially for runners.
For preventing of chafing and comfort, look for moisture-wicking fabrics such as Coolmax and Drifit. Stay away from sports bras that are made of 100% cotton because once you start sweating, they’ll stay wet.
If you need a lot of support, look for sports bras with wide shoulder straps and racer back straps.
Examine the seams of the sports bras. You'll want seamless fabrics, seams with rolled edges, or covered seams to help prevent chafing.
If you like bras with under wire, check that the under wire is completely covered so it will not gouge.
Make sure you try on several different size sports bras. The right size for you may not actually be the cup size that you normally wear. The bra is too big if the material wrinkles or bunches. And if the bra is cutting into your skin, then it's too small. If you can slide two fingers under the straps, then it's a good fit.
When you try on sports bras, jog in place or do jumping jacks to test their support before you buy. While you're bouncing up and down, the bra should minimize breast movement.
If you want to shop online, always look into the site's exchange and return policy. Titlenine.com has a good selection of bras and an excellent return policy, so you can return bras if they don't fit well.
After you find the perfect sports bra, don't hold onto it forever! You should replace bras after about 72 washes, when the elasticity is lost, or if your weight changes significantly. You can air dry bras to prolong the life of the moisture-wicking fabrics.