To measure at home, you’ll need two measurements: around your back and under your bust for your band size, and around your back over your nipples for your cup size. You’ll then subtract the difference. For example, if your bust measures 35 inches and your under-bust (or rib cage) 32 inches, you’ll be a 32C because 35 minus 32 equals 3, and that number corresponds to the letter “C” in the alphabet.
It’s totally normal and really common to have one breast that is bigger than the other. If the difference is significant enough that it makes bra shopping even more complicated than it already is, Cora Harrington, lingerie expert and author of the upcoming book In Intimate Detail: How to Choose, Wear, and Love Lingerie, suggests fitting to the larger breast. If you want, you can even out the appearance by adding a bra cutlet to the smaller breast, or getting a bra with removable pads and taking them out on the big side.
If your breasts are spilling out around the edges of the cup, they might be putting a lot of extra weight on the straps—and you may find yourself pulling the straps taut to hold them in check. Either way, your shoulders would probably benefit from larger cups.
Your straps could also be digging into your shoulders if your band is too loose, making it so your straps are doing all the work. Take a look behind you in the mirror: If your straps are pulled so tight that they’re yanking your strap up, it’s probably too big or is too stretched out to do its job.
Another tell is if the center gore, or the center panel on the front of your bra between the cups, is floating away. It should lay flat against the middle of your chest. And obviously, if the cups are gapping because your breasts are not filling them all the way, you may want to go down a cup size.
When you’ve got the right band size, you should be able to fit your finger between your back and the strap with only about an inch of stretch. Your band is too small if the underwire is squeezing or digging into your breast tissue. But looser is not better when it comes to support. Caldwell notes that most people think loose means more comfort (think: caftans or sweatpants), but that doesn’t work for bras. Remember that the band is what accomplishes most of the holding-up of the breasts, so a loose band that rides up between your shoulder blades will not provide the support you need and leave you less comfortable in the long run.