38A and 40AA are sister sizes — the same cup volume worn on a band that's 2 inches different, 38A being the snugger, more supportive option and 40AA the looser, roomier one.
38A and 40AA are sister sizes — the same cup volume worn on a band that’s 2 inches different, 38A being the snugger, more supportive option and 40AA the looser, roomier one.
If a fitter has ever told you to “try a sister size,” 38A and 40AA are the textbook example of what that actually means.
Why 38A and 40AA 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: 38A pairs a tighter, more supportive band with a smaller cup letter, while 40AA pairs a looser band with a bigger cup letter — both are carrying the identical amount of breast tissue, just distributed differently.
Online shoppers run into this constantly: two different brands list what should be the same size completely differently, and checking the sister size is often the fix.
Signs You Need the Snugger 38A Instead
Lean toward 38A 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 40AA Instead
Lean toward 40AA 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
People with a fuller ribcage or higher activity level often do better in the snugger band; people who find bands generally uncomfortable often prefer the looser one with the bigger cup.
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
Wireless options can work fine here as long as the band itself has real structure — wireless doesn’t have to mean unsupportive.
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
Use the Sister Size Calculator with your current band and cup — it applies this exact ±2 inch / ∓1 letter rule automatically so you don't have to do the math by hand.
The same math applies, but sports bras are built for compression and movement control, so many people size down in the band specifically for sports even if they wear the looser size everyday.
Often yes — very small or very large bands in a given cup letter tend to be less common in physical stores, so the sister size can be a useful workaround when your first choice is sold out.
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.
Mid-morning to early afternoon tends to be most representative for most people — very early morning or late evening can show minor temporary swelling that isn't your typical baseline.
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.
