38B vs 40A: Are They Sister Sizes? Same Cup Volume, Different Band Explained (2026)
Quick Answer: Yes — 38B and 40A are confirmed sister sizes. They hold approximately the same cup volume on different band sizes. Going up one band from 38 to 40 and dropping one cup letter from B to A follows the sister size rule exactly, preserving cup volume. The difference between them is entirely in the band: 38B is firmer for a narrower ribcage; 40A is wider for a broader one. Neither is correct for everyone — your underbust measurement decides which fits.
✅ This is a confirmed sister size comparison. Unlike same-letter comparisons such as 38B vs 40B — where the same letter on a larger band produces more cup volume — 38B and 40A follow the sister size rule and hold equivalent breast tissue volume. The band is the only meaningful difference between them. If your 38B is unavailable, 40A is the correct swap — not 40B.
⚡ Key Takeaways
- Sister sizes: 38B and 40A hold approximately equal cup volume — they belong to the same volume family.
- Band is the only real difference: 38B fits ~33–34″ (84–86 cm) underbust; 40A fits ~35–36″ (89–91 cm) underbust.
- 38B = firmer band, slightly deeper cup: More projection on a narrower, better-anchored frame.
- 40A = wider band, broader cup: Same volume distributed across a broader chest wall.
- Full sister size family: 34D — 36C — 38B — 40A.
- Correct substitution: When 38B is sold out, 40A is the right swap — not 40B.
- Support depends on band fit: On the correct ribcage, both sizes perform equally — measure your underbust first.
- 40A is rare: Most retailers stock 40-band sizes from C cup up. If 40A is hard to find, 38B is the closest available alternative.
- Underwire width differs: The 40A underwire is wider to match a broader chest wall — wearing it on a narrower ribcage means the wire will sit beyond your breast tissue boundary.
Why 38B and 40A Hold the Same Cup Volume
Cup letters in bra sizing are ratios, not fixed measurements. Each letter represents how far the bust extends beyond the underbust, expressed as an inch-per-letter scale. The principle behind sister sizing is straightforward: when the band increases by one size — adding approximately 2 inches of circumference to the base — dropping one cup letter offsets that gain, keeping the total enclosed volume roughly constant.
- 1″ = A | 2″ = B | 3″ = C | 4″ = D | 5″ = DD/E
A 38B means the bust is 2 inches larger than a 38-inch base — producing a bust of approximately 40 inches. A 40A means the bust is 1 inch larger than a 40-inch base — producing a bust of approximately 41 inches. The wider base of the 40A compensates for its smaller cup differential: a 1-inch gap on a 40-inch frame encloses approximately the same three-dimensional cup space as a 2-inch gap on a 38-inch frame. The letter changes; the volume doesn’t.
The critical distinction: this is not the same as comparing 38B to 40B. Moving to 40B while keeping the B letter would add one full cup size of volume on top of the wider band — a significantly larger bra. 40A is the correct sister of 38B, not 40B. See our cup size visuals page for a detailed visual breakdown of how volume changes across band and cup combinations.
Measurement Breakdown: 38B vs 40A
The table below shows how each size is built from real body measurements. The cup differential differs by letter — but the three-dimensional cup space each size encloses remains approximately equal.
| Size | Underbust (Band) | Bust (Fullest Point) | Cup Difference | Cup Volume |
|---|---|---|---|---|
| 38B | ~33–34″ (84–86 cm) | ~40″ (102 cm) | 2″ (5 cm) | ≈ Equal ↔ |
| 40A | ~35–36″ (89–91 cm) | ~41″ (104 cm) | 1″ (2.5 cm) | ≈ Equal ↔ |
The cup differential numbers differ — 2 inches for 38B versus 1 inch for 40A — but the wider circumference of the 40 band compensates, producing approximately equivalent enclosed volume. This is the mathematical foundation of the sister size principle: the ratio changes, the total cup space stays constant across sister pairs.
EU sizing: 38B = 85B; 40A = 90A. Both use different band and cup combinations but represent equal cup volume. Always verify with the specific brand’s conversion chart before ordering internationally. Use our international bra size charts for full reference.
The Sister Size Family: Where 38B and 40A Both Belong
Because 38B and 40A hold the same cup volume, they sit on the same rung of the same sister size ladder. Every size in the chain below holds approximately equal breast tissue volume — the band changes, the cup letter adjusts, but the total enclosed space stays constant throughout.
Moving left on the ladder tightens the band and deepens the cup; moving right loosens the band and widens the cup — volume stays approximately constant throughout. The most natural substitution pair is the immediate neighbour: 38B and 40A are adjacent entries and the most commonly swapped pair in this family. Explore the full chart on our sister sizes guide or generate your personal ladder with the sister size bra calculator.
Why This Family Is Relatively Short
Most sister size families extend five or more steps across the ladder. The 38B / 40A family is shorter because the A cup — a 1-inch differential — is the smallest standard cup letter. There is no 42AA in most mainstream sizing systems (though some brands carry AA), which means the chain ends at 40A rather than continuing outward. Moving inward toward firmer bands, 36C and 34D remain valid sisters with equal volume. The practical implication: if you’re a 40A and need a firmer band, 38B is your only immediately available sister size in most retailers. And if you’re a 38B and need a looser band, 40A is the only direct option — there’s nowhere further to go without leaving this volume family entirely.
Real Fit Differences Between 38B and 40A
Cup volume is theoretically equal — but the physical fit experience differs in every dimension except cup volume itself. Here’s what actually changes between the two sizes.
Band Fit: The Only Real Difference
A 38B band is engineered for a ribcage of approximately 33–34 inches. On that frame it sits level, anchors the cups without assistance from the straps, and delivers consistent support throughout the day. On a 35–36 inch ribcage, the same 38B band will feel uncomfortably tight — leaving marks under the arms, restricting deep breaths, and creating pressure across the back that worsens with the hours.
The 40A band is built for a 35–36 inch ribcage and delivers the same anchoring function on that wider frame. On a 33–34 inch ribcage, a 40A band will feel immediately loose — beginning to ride upward at the back within the first hour of wear, pushing the cups downward, and transferring all breast support to the shoulder straps. In a B cup this creates noticeable strap dig-in and posture strain more quickly than in larger cup sizes, because the straps are being asked to do a job that requires a properly anchored band.
Underwire Width and Breast Root
The 38B underwire is set for a narrower chest wall — the wire channel begins and ends closer to the centre of the chest, and the cup curves forward more steeply from a narrower base. The 40A underwire is wider, designed for breast tissue that roots more broadly across a larger ribcage.
Wearing a 40A when you have a 38-inch ribcage means the wire ends will extend onto bare ribcage beyond your natural breast tissue boundary — the most reliable cause of side-poking and underarm irritation. Wearing a 38B when your ribcage is 35–36 inches means the underwire will press into the side of the breast tissue rather than resting cleanly on the ribcage beneath it. Both problems are band-width problems, not cup problems. See our bra fit problems guide for a full diagnosis checklist.
Cup Shape and Projection
On a 38-inch frame, the B cup sits with slightly more forward projection — the breast tissue is supported centrally on a tighter base, creating a more defined shape. On a 40-inch frame, the A cup distributes the same volume more broadly, with the cup geometry spreading laterally to follow a wider chest wall. Neither shape is better; both are correct for their respective frame widths. Women with relatively close-set breast tissue often find the 38B geometry matches their shape more naturally; women with wider-set, flatter breast tissue frequently find the 40A underwire channel follows their natural boundary more accurately.
Who Should Choose 38B?
- Your underbust measures approximately 33–34 inches (84–86 cm) snugly beneath the breasts.
- Your bust at the fullest point is approximately 40 inches (102 cm) — a 2-inch differential on this frame.
- You’ve tried a 40A and found the band riding up at the back by midday, straps digging in, or cups dropping forward.
- Your breast tissue sits relatively close-set or projects forward on a narrower chest wall — the 38B underwire follows this contour correctly.
- Your current 38B band feels correct but cups overflow slightly → try 38C on the same band, not 40A.
- 38B is widely stocked and readily available across mainstream and specialist retailers.
Confirm your correct fit at our how to know your bra fits page.
Who Should Choose 40A?
- Your underbust measures approximately 35–36 inches (89–91 cm) snugly beneath the breasts.
- Your bust at the fullest point is approximately 41 inches (104 cm) — a 1-inch differential on this wider frame.
- You’ve tried a 38B and found the band constricting, leaving deep marks under the arms, or restricting breathing from the first wear.
- Your breast tissue spreads widely and sits more laterally across a broader chest wall — the 40A underwire width matches this distribution better.
- Your current 40A band feels correct but cups overflow → try 40B on the same band, not 38B.
- 40A can be difficult to find in mainstream stores — if unavailable, 38B is your closest same-volume alternative.
Use our breast shape identifier and size charts to verify your best fit.
🛍️ Best Bras for 38B and 40A — Our Top Picks
B cup wearers on a 38 band and A cup wearers on a 40 band share equal cup volume but need bras with the correct band construction for their specific ribcage width. A well-made bra in either size should deliver clean cup shaping, a band that stays level all day, and straps that provide guidance rather than carrying the full load. These two highly rated options on Amazon are available in both 38B and 40A.
Warner’s Women’s Elements of Bliss Wire-Free Contour Bra #1269
One of the most reviewed wire-free bras on Amazon, and a particularly reliable choice for this sister size pair. The contour foam cups are pre-shaped to each specific letter size — meaning the 38B and 40A versions are built with the correct cup geometry for their respective band widths, not just the same cup shell relabelled.
The wide multi-hook back band is structured enough to stay level on both a 38-inch and a 40-inch ribcage, and the tagless construction eliminates the underarm irritation that wider bands sometimes cause. An excellent option for women switching between 38B and 40A to find their correct ribcage size, since the cup performance will be consistent across both.
Available in: 38B, 40A, and full range through 40DD
View on Amazon →
Maidenform Women’s Comfort Devotion Extra Coverage Underwire Bra #09404
A consistently top-selling underwire bra that handles extended band sizes well — the 38 and 40 band versions both carry enough structure to prevent the back-riding that affects wider bands on active days. The memory foam padding conforms precisely to the shape of B and A cup breast tissue, delivering natural rounded shaping without the push-up effect that flattens or distorts at smaller cup volumes.
The full-coverage cup height respects the correct cup geometry for both the 38B and 40A letter sizes, and the multi-hook back provides long-term band adjustability. An excellent all-day underwire choice for either size in this sister pair.
Available in: 38B, 40A, and extended range through 42DDD
View on Amazon →ℹ️ As an Amazon Associate, Bra Calculator earns from qualifying purchases. Product availability and pricing subject to change.
The Sister Size Rule Applied to 38B and 40A
| Comparison Type | Example | Same Volume? | Relationship |
|---|---|---|---|
| Band up + Cup letter down | 38B → 40A | ✅ Yes | Sister sizes — equal volume, looser band |
| Band down + Cup letter up | 38B → 36C | ✅ Yes | Sister sizes — equal volume, firmer band |
| Band up + Cup letter down | 40A → 38B | ✅ Yes | Sister sizes — equal volume, firmer band |
| Band up, same cup letter | 38B → 40B | ❌ No | 40B is larger — one full cup above 38B |
| Band down, same cup letter | 40A → 38A | ❌ No | 38A is smaller — different volume family |
| Same band, cup up | 38B → 38C | ❌ No | 38C is one full cup larger on the same band |
| Same band, cup down | 40A → 40AA | ❌ No | 40AA is one cup smaller (rarely stocked) |
The only correct band-change substitute for a 38B is 40A (looser band, same volume) or 36C (firmer band, same volume). Moving to 40B adds both a wider band and more cup volume simultaneously — never a sister size swap. When 38B is out of stock, 40A gives you the closest possible fit on a looser band; when 40A is out of stock or unavailable, 38B is the correct same-volume alternative on a firmer band.
Quick Bra Fit Test: 5 Checks for 38B and 40A Wearers
Whether you’re currently in a 38B or a 40A, run through these five checks to confirm your bra is fitting your body correctly — or whether the sister size switch would make a real difference to your all-day comfort and support.
Lean forward and scoop all breast tissue fully forward and upward into the cups. After scooping, overflow at the top or sides means the cup is too small — go up one cup letter on the same band (38B → 38C or 40A → 40B). Wrinkling or gaping fabric at the top of the cup means cup too large — go down one letter on the same band. Never change the band to fix a cup overflow or gaping problem. Band and cup problems have different solutions: always fix the cup first before reassessing the band.
The gore must lie completely flat against the sternum throughout the entire day — not just when you first put it on in the morning. For 38B and 40A wearers, a floating gore is typically a sign that the cup is too small on the current band. Go up one cup letter on the same band (38B → 38C or 40A → 40B) before making any other change. If the gore lies flat but the cups wrinkle at the top, the cup is too large — go down one letter or switch to your sister size on the adjacent band (40A → 38B or 38B → 36C).
The underwire must sit fully on the ribcage and encircle all breast tissue at every point. For this sister pair specifically, underwire width is the tell: if you’re in a 40A and the wire ends extend onto bare ribcage beyond your breast tissue boundary — the underwire is too wide for your chest wall. Try 38B. If you’re in a 38B and the wire presses into the soft tissue at the sides of the breast rather than resting on the ribcage beneath it — the underwire is too narrow. Try 40A. Both problems are band-width problems; the cup volume is correct in both cases.
On the loosest hook, slide two fingers under the back band with firm, consistent resistance. For a 38B: if your whole hand slides under easily, your underbust is likely closer to 35–36 inches and you need a 40 band — try 40A (same cup volume as your 38B). For a 40A: if one finger won’t fit comfortably on the loosest hook from day one, your underbust likely measures 33–34 inches and the 38B will fit and support far better. The band, not the straps, carries approximately 80% of breast support — a loose band on either sister size will make that weight felt in your shoulders by midday.
Raise both arms overhead, twist from side to side, and walk briskly for 60 seconds. The band must stay completely level front and back, cups must remain fully in position, and straps must stay on shoulders without digging in. Any back-riding of the band during movement is a definitive sign the band is too loose for your ribcage — switch to the sister size with the firmer band (40A → 38B, or 38B → 36C). Do not tighten straps to compensate for a riding band; this solves nothing and creates shoulder and neck strain.
Still unsure whether 38B or 40A — or a completely different size — is right for your measurements? Get a precise answer based on your actual underbust and bust numbers.
Try the AI-Powered Bra Size Calculator →38B vs 40A: Full Side-by-Side Comparison
| Feature | 38B | 40A |
|---|---|---|
| Band Size | 38 (33–34″ / 84–86 cm underbust) | 40 (35–36″ / 89–91 cm underbust) |
| Cup Letter (US/UK) | B — 2″ (5 cm) over band | A — 1″ (2.5 cm) over band |
| EU Approximate | 85B | 90A |
| Cup Volume | ≈ Equal ↔ | ≈ Equal ↔ |
| Bust Measurement | ~40″ (102 cm) | ~41″ (104 cm) |
| Sister Size? | ✅ Yes — confirmed sister sizes, same volume family | |
| Underwire Width | Narrower — closer-set breast root | Wider — broader breast root |
| Cup Shape | Slightly deeper, more forward projection | Broader, more lateral spread |
| Band Tightness | Firmer — correct for 33–34″ ribcage | Looser — correct for 35–36″ ribcage |
| Full Sister Family | 34D — 36C — 38B — 40A | |
| If cups too small | Try 38C (same band, cup up) | Try 40B (same band, cup up) |
| If cups too large | Try 38A (same band, cup down) | Try 40AA if available (same band, cup down) |
| If band too tight | Try 40A (sister size — same volume) | No standard sister size outward — volume family ends here |
| If band too loose | Try 36C (sister size — same volume) | Try 38B (sister size — same volume) |
| Best For | Underbust ~33–34″, narrower chest wall | Underbust ~35–36″, wider chest wall |
| Availability | Very widely stocked — one of the most common sizes | Limited — specialty retailers and online; 40 band in A cup is rarely stocked mainstream |
People Also Ask: 38B vs 40A — Answered
Are 38B and 40A the same size?
Yes — 38B and 40A are confirmed sister sizes, meaning they hold approximately the same cup volume on different band sizes. The 38B has a firmer band suited to a 33–34 inch ribcage with a slightly deeper cup. The 40A has a wider band for a 35–36 inch ribcage with a broader, shallower cup. Neither holds more cup volume — the band is the only meaningful difference between them.
Which is bigger — 38B or 40A?
Neither is bigger in cup volume — both enclose approximately the same amount of breast tissue. The 40A bra is physically wider because of its larger band, but the enclosed cup space is equivalent to the 38B. The 38B cup projects slightly more forward on its narrower base; the 40A distributes the same volume more broadly on a wider frame. Volume is equal; fit depends entirely on your underbust measurement.
Can I substitute 40A for 38B?
Yes — 40A is the correct sister size substitute when your 38B is unavailable. Cup volume will be approximately equal. The 40A band is 2 inches wider, so it will feel noticeably looser on a 33–34 inch ribcage. If the looser band causes the bra to ride up your back during the day, the 40A band fit is not correct on your frame regardless of the cup volume match — return to 38B or try 36C (firmer band, same volume).
Why do 38B and 40A hold the same cup volume despite different letters?
Because cup letters measure a ratio — the difference between bust and underbust — not a fixed volume. A B cup means the bust is 2 inches larger than the underbust on any band; an A cup means it’s 1 inch larger. On a 40-inch frame, a 1-inch gap creates approximately the same three-dimensional cup space as a 2-inch gap on a 38-inch frame, because the wider base compensates for the smaller differential. This is the mathematical foundation of sister sizing and applies universally across all size families.
My 38B band feels too loose — should I try 36B or 36C?
Try 36C — not 36B. A 36C is the sister size of 38B: it gives you the next band size down while preserving the same cup volume. Moving to 36B would give you a firmer band but significantly less cup volume — you’d experience cup gaping immediately. The rule: when you go down a band size to get a firmer fit, always go up one cup letter to maintain the same volume. 38B → 36C is the correct move; 38B → 36B loses you a full cup size.
My 38B cups feel too small but the band fits — should I try 38C or 40A?
Try 38C — same band, next cup letter up. Do not switch to 40A: the 40A is the sister size of 38B, meaning it holds the same cup volume as your current too-small 38B on a larger band. Moving to 40A gives you a looser band with no additional cup space — it solves neither problem. Fix cup fit by changing the cup letter on the same band (38B → 38C); fix band fit by switching to a sister size (38B → 40A or 36C).
Is 40A a hard size to find?
Yes — 40A is one of the less commonly stocked bra sizes. Most mainstream retailers and specialist brands stock the 40 band starting from C cup, because the combination of a wider band with an A cup is statistically less common. Online retailers typically carry the broadest 40A selection. If you genuinely measure as a 40A but cannot find the size, 38B is the correct same-volume alternative with a firmer, slightly more widely available band.
What is the full sister size family for 38B and 40A?
The full sister size family is 34D — 36C — 38B — 40A. All four sizes hold approximately equal cup volume on progressively wider band sizes. Moving left tightens the band and deepens the cup; moving right loosens the band and widens the cup. The chain ends at 40A because the A cup is the smallest standard letter — there is no widely available 42AA that extends the family further outward.
Does band size change cup shape even when volume is equal?
Yes — and this is particularly noticeable between 38B and 40A. The 38B cup is constructed for a narrower chest wall: the underwire is set closer together and the cup projects more steeply forward, suited to close-set or projected breast tissue. The 40A cup is broader, with the underwire farther apart and a more lateral spread, suited to breast tissue that roots broadly across a wider ribcage. Same volume — genuinely different cup geometry. Choosing the right sister size for your specific breast shape, not just your ribcage measurement, produces the best fit result.
How do I know my correct bra size?
Measure your underbust snugly for your band and your bust at the fullest point for your cup. Subtract underbust from bust — each inch equals one cup letter: 1 inch = A, 2 inches = B. Verify fit with five checks: band level all around, flat gore throughout the day, underwire on ribcage matching your breast root width, two fingers under back band on loosest hook with firm resistance, no cup movement during activity. Use our bra size chart calculator for a precise, personalised result.
