38C and 40B are sister sizes — the same cup volume worn on a band that's 2 inches different, 38C being the snugger, more supportive option and 40B the looser, roomier one.
38C and 40B are sister sizes — the same cup volume worn on a band that’s 2 inches different, 38C being the snugger, more supportive option and 40B the looser, roomier one.
38C and 40B solve the same fit problem two completely different ways: one leans on a snugger band, the other on a looser one.
Why 38C and 40B Hold the Same Volume
It isn’t a coincidence that these two sizes fit similarly — it’s built into how the sizing system works. Two sizes are “sisters” whenever band inches and cup letters move by the same step in opposite directions.
In practice: 38C pairs a tighter, more supportive band with a smaller cup letter, while 40B 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 38C Instead
Lean toward 38C 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 40B Instead
Lean toward 40B 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
If straps constantly slip off your shoulders, the tighter-band option usually helps more than a bigger cup would — bands do most of the actual support work, not straps.
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
Weight fluctuation of even a few pounds can nudge a band size, especially at the boundary between two sizes — this isn’t a flaw in the measuring process, just how bodies naturally vary.
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
If you're unsure, the snugger band (38C) is usually the safer default, since bands stretch out slightly with wear and wash over time — starting snug tends to age better than starting loose.
Check the tag inside the band — the number is the band size and the letter(s) after it are the cup. If there's no tag, the fit clues (band riding up vs. cup gaping) will tell you which direction to adjust.
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 — 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.
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.
