Bra sizes are made up of two components: An even number that represents the band size (32, 34, 36, etc.), and a letter that indicates the cup size (A, B, C, etc.) determined by the size of the breast itself.
There are many competing methods for determining the best bra size to select. What follows is the method with which we have had the most success.
Always measure while wearing your own best fitting bra - This should NOT be a sports bra, but rather a "lingerie" style bra, as you would typically wear underneath a blouse.
To determine the band size:
Measure snugly around your ribcage, directly under your breasts. Add 3 inches to this measurement. For example, if your ribcage is 31 inches, your band size is 34. If your result is an odd number, go UP to the next band size.. For example, if your rib cage is 32 inches, 32+3=35, round 35 up to a 36 band size.
To determine the cup size:
Measure loosely around the fullest part of your bust. Subtract the band size from this measurement. Use this number to determine your cup size according to the table below.
Example: if your ribcage measurement is 31, and your bust measurement is 37, your bra size is a 34C: 31+3=34, so 34 is your band size. Since 37-34= 3, you've got a C cup.
Fit Check List:
Compression uses elastic support fabrics to hold breasts closer to the body. Reduces the amount of bounce, but may produce a “uniboob” appearance which some women object to.
Encapsulation is another method of reducing bounce, encapsulation typically uses molded, often semi-rigid cups to contain the breasts. The shape of the cup, and the way it's built into the bra, limits movement of the breasts.
Many bras use a combination of compression and encapsulation to support breasts.
Underwire/frame support system
Stiffening components underneath the breast hold it up and provide shaping. May also contribute to the overall support of the bra.
Straps support the weight of the breasts by cantilevering it off the shoulders, much like a suspension bridge. Larger breasts require broader, non-stretchy straps.
Many A/B bras are designed to pull over the breasts. When there's a lot of difference between band size and bust size, it's difficult to pull on a bra that's suitably sized in the band, so most C/D/DD bras have a front or rear closure. To prevent chafing, the closure should be covered so that it's invisible to the skin.