How Bra Sizing Works
Band size and cup size are not independent measures, they are interrelated.
The band size is expressed as an even number of inches; it represents the measurement of your chest just below your breasts.
The cup size is a letter that represents the difference between your band size and the measurement of your body at the fullest part of your breasts. Each increase in letter represents an increase of 1" in the difference between the band size and the measurement of your breasts.
For example, a 38C means a chest measurement of 38" and a breast measurement of 41". The difference between the two measurements is 3" which is a C cup. A 38D would be a chest measurement of 38" and a breast measurement of 42". The 4" difference in the two measurements is a D cup. And so on.
The tricky part is because band size and cup size are interrelated there is no standard size for a D cup, or any cup size for that matter. The cup in a 40D is larger than the cup in a 38D. Got that?
To further confuse things, in standard U.S. sizing there are no E or F cups. A breast measurement 5" larger than the band size is a DD and a breast measurement 6" larger than the band size is a DDD. Some European brands do have E and F cups. And there is some variance in sizing among the brands.
We want to make it simpler for you, so we've created a few tools and charts to help.
Bra Size Calculator
European Band Size Conversions for Bras
Cup size is the same between U.S. and Europe/France. "A" cups (U.S.) = "A" cup in Europe/France; B=B, C=C, etc with some exceptions for "DD" and "DDD" cups. Many European brands (and some American brands) use the lettering "E" for a DD cup size and "F" for a DDD cup size. The cup size guide above should help you determine which cup size is appropriate for you.
Sister sizing is a technique used by bra fitters when they just can't find the right bra for you with your natural size. Sister sizing is not as ideal as the perfect fitting bra, but you may try it if you are struggling to get a bra that you like in the size that you need. Take your normal size bra (let's use 32F as our example) the sister sizes are 34E and 30G. If you go up a band size, come down a cup size. Likewise if you go down a band size, come up a cup size.
Consider Sister Sizing only when you absolutely can not find the perfect fit from the bra that you want. In the chart below, find your size, then look at the sizes directly to the left and right of your size - those are your sister sizes.