If your usual store is out of stock in 38B, 40A is the size built to hold the exact same volume. Worth trying before you give up and leave empty-handed.
Why 38B and 40A Hold the Same Volume
Every 2-inch step up in band size shifts the cup ladder by exactly one letter in the opposite direction. That’s not a coincidence: both sizes are describing the same physical volume, just measured against a different band width.
In practice: 38B pairs a tighter, more supportive band with a smaller cup letter, while 40A pairs a looser band with a bigger cup letter. Both are carrying the identical amount of breast tissue, just distributed differently.
It also explains why a bra you loved for years can suddenly feel wrong after weight changes, pregnancy, or just time. You may have simply shifted from one of these sizes toward the other without realizing it.
Signs You Need the Snugger 38B Instead
Lean toward 38B 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 40A Instead
Lean toward 40A 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
If you’re shopping online and can only return one, lean toward the snugger band. A too-loose band is a much more common (and more uncomfortable) fit mistake than a slightly loose 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
Full-coverage and plunge styles with real structure hold up better than soft, unlined designs at this range.
This Can Change Over Time
Body measurements can shift slightly with the seasons for some people. A bra that fit perfectly in summer can feel different after a few months, which is normal and worth a quick re-check rather than assuming something’s wrong.
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.
