If a fitter has ever told you to “try a sister size,” 34A and 36AA are the textbook example of what that actually means.
Why 34A and 36AA Hold the Same Volume
The underlying rule is simple arithmetic: plus 2 inches of band always pairs with minus 1 cup letter, and vice versa, because both numbers are just two different ways of describing one physical size.
In practice: 34A pairs a tighter, more supportive band with a smaller cup letter, while 36AA pairs a looser band with a bigger cup letter. Both are carrying the identical amount of breast tissue, just distributed differently.
This comes up constantly when a store is sold out of one size but has the other in stock. It’s genuinely worth grabbing the sister size rather than leaving without trying anything on.
Signs You Need the Snugger 34A Instead
Lean toward 34A if the straps are doing most of the lifting and digging into your shoulders because the band isn’t holding its share. A band that isn’t doing its job shows up as strap and shoulder problems long before it shows up as an obvious “this is loose” feeling.
Signs You Need the Roomier 36AA Instead
Lean toward 36AA if the center gore (the bit between the cups) doesn’t sit flat against your sternum. Cup problems are usually easier to spot than band problems, but they get blamed on “bad luck with bra shopping” far more often than they should.
Which One Should You Try First
There’s no universal “better” choice here. Brand and style matter as much as the size label. The honest answer is to try both if you can and see which band tension your ribcage actually prefers.
Not sure which of these two is actually your size? Run your measurements through the Sister Size Calculator or the Main Bra Calculator- both are free, private, and take under a minute.
Style Considerations
Balconette and demi styles tend to work well here since they don’t rely on excess fabric to create shape.
This Can Change Over Time
If you’ve changed your exercise routine significantly, muscle and tissue distribution can shift enough to affect band fit specifically, even without a change in overall weight.
Shopping Tips
Reading recent reviews that mention true-to-size feedback for the exact style (rather than the brand generally) is one of the more reliable ways to predict fit before buying online.
