Finding the proper fit for a bra can be difficult, especially since the relationship between one's band size and one's cup size can be confusing. Luckily, Calvin Klein provides specific instructions for how to measure yourself for their bras. The company uses a method popular among other makers of women's lingerie, too.
Measure your band size. Standing up straight, use a measuring tape to find the number of inches around your rib cage. Arrange the measuring tape just below your breasts, where the bra band will sit.
Adjust the number. If your measuring tape says 33 inches or less, add five inches to the number. If the measuring tape says 34 inches or more, add four inches. For example, if your rib cage is 35 inches, add 4 to make 39. There is a controversy in the lingerie world whether one should add these numbers, but Calvin Klein is in the camp that does. So calculate this way for their brand.
Adjust again. If your new number is odd (for example, 39), add one to make it an even number (in this case, to 40). This final number is your "band size." In the above example, you'll be a 40A or 40B or 40C or 40D.
Measure your cup size. Standing straight, use your measuring tape to find the number of inches around the fullest part of your bust, then subtract your band size. For example, if your band size is 40 and your bust measures 44 inches at its fullest, the important number for your cup size is four.
Check these conversions for your cup size: one=A cup, two=B cup, three=C cup, four=D cup, five=DD cup, six=DDD cup. So, our example of a woman who measures 35 inches below the bust and 44 inches at the fullest point of the bust would wear a Calvin Klein size 40D bra.
Measure yourself every six months, or if you gain or lose weight, since your bra size can change quickly.
Regardless of what your tape measure says "should" be your size, if the actual bra is too tight, or if you don't fill the cups, find a different size that actually meets your needs. Remember that if you go down to a smaller band size, your cup size will increase, while if you go up to a larger band size, your cup size will decrease.