36A and 38AA are sister sizes — the same cup volume worn on a band that's 2 inches different, 36A being the snugger, more supportive option and 38AA the looser, roomier one.
36A and 38AA are sister sizes — the same cup volume worn on a band that’s 2 inches different, 36A being the snugger, more supportive option and 38AA the looser, roomier one.
If a fitter has ever told you to “try a sister size,” 36A and 38AA are the textbook example of what that actually means.
Why 36A and 38AA 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: 36A pairs a tighter, more supportive band with a smaller cup letter, while 38AA pairs a looser band with a bigger cup letter — both are carrying the identical amount of breast tissue, just distributed differently.
Fitters bring this up so often precisely because most people have never been told that two very different-looking size labels can describe the same body.
Signs You Need the Snugger 36A Instead
Lean toward 36A if the band rides up in the back within an hour of putting it on. 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 38AA Instead
Lean toward 38AA if there’s visible spillage at the top or sides of the cup by midday. 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
A quick test: if your current bra’s band rides up your back within an hour, go for the tighter band. If the band feels fine but the cups gape or wrinkle, the looser-band option is worth trying.
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
T-shirt style molded cups give the smoothest look under fitted clothing if that’s a priority alongside fit.
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 (not just the brand generally) is one of the more reliable ways to predict fit before buying online.
FAQ
Most do, since the underlying math is standard across the industry, but cut and construction still vary by brand and style — always sanity-check against that brand's own size chart.
Often yes, especially for underwire swim tops built like bras, but swimwear sizing is less standardized across brands than everyday bras, so treat it as a starting point, not a guarantee.
Not identical — the cup volume is the same, but a tighter band feels different against your ribs than a looser one. Most people have a genuine preference once they try both.
That usually means the starting measurement was slightly off rather than the sister-size logic being wrong — re-check your underbust and bust measurement with the main calculator before trying a third size.
Yes — measuring while slouching or with unnatural posture can shift the numbers slightly; stand naturally with normal, relaxed posture for the most accurate result.
Often yes — sports bras use different construction (more compression, less structured cup shaping) that can make the same labeled size feel noticeably different from an everyday style.
Yes, for most people — weight change, age, hormonal shifts, and even different laundry habits can all move the number gradually, which is why periodic re-measuring matters more than getting it right once.
