40A vs 40B Bra Size: What Is the Real Difference? (2026 Fit Guide)
Quick Answer: No — 40A and 40B are not the same size. Both sit on the same 40-inch band, fitting the same ribcage of approximately 35–36 inches (89–91 cm) underbust, but they hold different cup volumes. A 40B contains one full cup size more breast tissue than a 40A — approximately 150–175 ml more per cup. The band is identical; the cup depth, projection, and enclosed volume are genuinely different.
The decision between them comes down to one measurement: your bust-to-underbust differential. A 1-inch gap points to 40A; a 2-inch gap points to 40B. These are distinct sizes on the same frame — not interchangeable alternatives.
⚡ Key Takeaways
- Not the same size: 40A and 40B share one band but hold different cup volumes — 40B is one full cup larger.
- Same 40 band: Both fit an underbust of ~35–36″ (89–91 cm) — a wider band most commonly found in full-figure and plus-size ranges.
- 40A cup: 1″ bust-to-underbust differential (~41″ / 104 cm bust) — shallow cup, modest volume.
- 40B cup: 2″ bust-to-underbust differential (~42″ / 107 cm bust) — deeper cup, ~150–175 ml more volume per breast than 40A.
- Not sister sizes: Sister of 40A = 38B (tighter band). Sister of 40B = 38C (tighter band). They are separate volume families.
- UK/EU alignment: US 40A = UK 40A = EU 90A; US 40B = UK 40B = EU 90B. No naming divergence at A and B cups.
- Band construction identical: Underwire width, hook-and-eye length, and band tension are the same in both — only the cup arc height and depth differ.
- 40A is the smallest standard cup on a 40 band — availability can be limited in mainstream retail even at wider bands.
- If cups overflow in 40A → try 40B: Same band, one cup deeper. Do not switch bands to fix a cup problem.
- If cups gap in 40B → try 40A: Same band, one cup shallower. Again — keep the band, change only the cup.
40A vs 40B: The Core Difference Explained
Both 40A and 40B are built on exactly the same 40-inch band. The band’s circumference, hook-and-eye length, underwire span across the chest wall, and strap attachment points are all identical between the two sizes. On the correct ribcage — approximately 35–36 inches (89–91 cm) underbust — both will anchor with equal band tension and provide equivalent structural support.
The cup is the only variable. Cup letters represent the difference between the fullest bust and the underbust — each inch of gap equals one cup letter:
- 1″ difference = A cup
- 2″ difference = B cup
- 3″ difference = C cup
- 4″ difference = D cup
A 40A means your bust is 1 inch larger than your underbust on a 40-inch frame — a shallow cup with modest enclosed volume. A 40B means your bust is 2 inches larger on the same frame — a cup that is meaningfully deeper, wider in its arc, and holds substantially more breast tissue. The difference translates to approximately 150–175 ml more volume per cup in the 40B — roughly the volume of a small yogurt pot per side, and across both cups a total of 300–350 ml of additional enclosed space. That is a visible, functional difference — not a minor adjustment.
The Volume Difference Visualised
40B holds ~150–175 ml more per cup than 40A — same band, genuinely different cup volumes
Switching from 40A to 40B is not a sister size swap — it changes cup volume while keeping the band fixed. Sister sizing only applies when you change both the band and the cup letter simultaneously so that the total enclosed volume stays constant. 40A and 40B do not share the same volume, so they cannot be sister sizes.
Measurement Breakdown: 40A vs 40B
The table below shows exactly how each size is derived from real body measurements and what that means in terms of cup volume and international equivalents.
| Measurement | 40A | 40B |
|---|---|---|
| Underbust (band) | ~35–36″ / 89–91 cm — identical for both | |
| Fullest bust | ~41″ (104 cm) | ~42″ (107 cm) |
| Bust–band differential | 1″ (2.5 cm) — A cup | 2″ (5 cm) — B cup |
| Cup volume (per breast) | ~550 ml | ~700 ml (+150–175 ml over 40A) |
| UK equivalent | 40A | 40B |
| EU equivalent | 90A | 90B |
| AU / NZ equivalent | 22A | 22B |
International naming is straightforward at A and B cups — unlike the DDD/F divergence above DD, cup letters A and B are fully consistent across US, UK, EU, and AU systems. US 40A = UK 40A = EU 90A. US 40B = UK 40B = EU 90B. Shop any brand’s 40A or 40B with full confidence that the cup letter refers to the same differential regardless of country of origin. Use our international bra size charts for full cross-system reference.
Sister Sizes: 40A and 40B Each Have Their Own Volume Family
Because 40A and 40B hold different cup volumes, they belong to entirely separate sister size families. Their chains do not overlap. Understanding this is essential — particularly for wider-band sizes where stock availability often makes a sister size swap the only practical option.
Sister Size Family for 40A
All sizes below hold the same cup volume as 40A on different band widths:
If your 40A band feels too loose or rides up during the day, try 38B — same cup volume in a firmer, better-anchored band. At A cup volume, a loose band transfers the modest breast weight to the straps surprisingly quickly. If the 40A band pinches, try 42AA where available. Important: 38B is the sister of 40A — not 38A. 38A is a smaller cup on a tighter band and reduces your cup volume.
Sister Size Family for 40B
All sizes below hold the same cup volume as 40B — entirely separate from the 40A volume family above:
If your 40B band feels too loose, try 38C — same cup volume in a tighter band that anchors more securely and delivers noticeably better lift. If the 40B band pinches, try 42A for the same cup volume with a wider, more relaxed band. Explore complete chains on our sister sizes guide or use the sister size bra calculator to generate your personal ladder.
The Same-Band Cup Rule Applied to 40A and 40B
| Comparison Type | Example | Same Volume? | What Changes |
|---|---|---|---|
| Same band, cup up | 40A → 40B | ❌ No | Cup volume increases by one full size — band unchanged |
| Same band, cup down | 40B → 40A | ❌ No | Cup volume decreases by one full size — band unchanged |
| Band down + Cup up (sister of 40A) | 40A → 38B | ✅ Yes | Tighter band, same cup volume as 40A |
| Band up + Cup down (sister of 40A) | 40A → 42AA | ✅ Yes | Wider band, same cup volume as 40A |
| Band down + Cup up (sister of 40B) | 40B → 38C | ✅ Yes | Tighter band, same cup volume as 40B |
| Band up + Cup down (sister of 40B) | 40B → 42A | ✅ Yes | Wider band, same cup volume as 40B |
| Same band, two cups up | 40A → 40C | ❌ No | Cup volume increases by two full sizes |
| Same band, same letter | 40A → 40A | ✅ Yes | Identical — same band, same cup, same volume |
The governing rule: any change to the cup letter on the same band always changes volume. Volume is only preserved across a band change when the cup letter adjusts by one letter simultaneously — that is what sister sizing is. On the 40 band, every step from A to B to C to D represents a genuine, measurable increase in cup volume that changes how the bra fits, feels, and performs.
Real Fit Differences Between 40A and 40B
Cup Depth, Projection, and Shape
The most immediately noticeable difference between 40A and 40B is cup depth. A 40A cup is engineered for very modest breast tissue on a wide frame — the cup is shallow, projecting only slightly from the band, with a small internal arc. For someone with a true 40A measurement, this shallow construction fits correctly and comfortably. For someone whose bust genuinely measures 2 inches beyond the underbust (40B territory), the 40A cup simply does not have enough internal space — breast tissue will overflow at the top, sides, or both.
The 40B cup is noticeably deeper — the cup panels are wider and taller, the internal arc projects further forward, and the total enclosed space is around 150–175 ml larger per side. This is the construction required for a 2-inch differential on a wide 40-inch frame. At a 40-inch band with moderate bust projection, the B cup is the starting point for genuine containment rather than minimal coverage.
Shallow Cup Challenges on a Wide Band
One factor specific to the 40 band is that the underwire is proportionally wider than on narrower bands — built to span a broader chest wall. Both 40A and 40B share this wider wire. For women with shallow or widely-spaced breasts at A cup volume, the 40A underwire may still extend past the breast root onto bare ribcage — a fit issue that can make the 40 band feel wrong even when the underbust measurement points there. In this scenario, the sister size 38B often resolves the wire-width problem while preserving cup volume on a narrower frame.
T-Shirt Silhouette and Daily Wearability
Both 40A and 40B are shallow-to-moderate cup sizes that work well under fitted T-shirts and everyday clothing without visible bulk. The 40A creates a minimal projection with a flat-to-the-chest silhouette; the 40B creates a gentle, natural curve while remaining T-shirt friendly. At neither size does projection become an issue under standard clothing — the main daily wearability distinction is cup containment, not visibility.
Bralette and Soft-Cup Compatibility
Both 40A and 40B are among the most bralette-compatible sizes in the wider-band range. Soft-cup, stretch-fabric, and wire-free styles generally work well at both A and B cup volumes on a 40 band, where breast tissue weight is modest enough that structured underwire is optional rather than necessary for most daily activities. Women who prefer bralettes and wire-free styles will find extensive options at both sizes.
Who Should Wear 40A?
- Underbust measurement approximately 35–36 inches (89–91 cm) snugly measured just beneath the bust.
- Bust measurement at the fullest point of approximately 40.5–41.5 inches (103–105 cm) — a 1-inch differential.
- Breast tissue is modest in volume and sits relatively close to the chest wall with minimal forward projection.
- Previously tried 40B and found the cups too large — wrinkling, gaping, or cups that do not contact breast tissue across the full cup surface after thorough scooping.
- Breasts have a shallow or “east-west” shape — tissue spreads laterally rather than projecting forward — making a shallow cup geometry a better fit regardless of projection measurement.
- If 40A cups overflow or gore floats: move up to 40B on the same band — do not change the band.
- If 40A band rides up: move to 38B (sister size — same cup volume, tighter band).
- UK/EU sizing: Shop for 40A / EU 90A — labels align directly.
Verify your fit with the five-point check on our how to know your bra fits page.
Who Should Wear 40B?
- Underbust measurement approximately 35–36 inches (89–91 cm) snugly measured just beneath the bust.
- Bust measurement at the fullest point of approximately 41.5–42.5 inches (105–108 cm) — a 2-inch differential.
- Previously tried 40A and found the cups too small — overflowing at the top or sides, a persistently floating centre gore, or underwire arc sitting on breast tissue rather than fully beneath it.
- Breast tissue projects more noticeably forward from the chest wall or has a rounder, fuller shape that requires more cup depth for complete containment.
- If 40B cups overflow: move up to 40C on the same band — do not change the band.
- If 40B cups gap or wrinkle: move down to 40A on the same band.
- If 40B band rides up: move to 38C (sister size — same cup volume, tighter band).
- UK/EU sizing: Shop for 40B / EU 90B — labels align directly.
Use our breast shape identifier and size charts to confirm the best style for your shape on a 40 band.
🛍️ Best Bras for 40A and 40B — Our Top Picks
On a 40 band at A and B cup volume, the key requirements are a wide, well-anchored band that stays level on a broader ribcage, a cup depth appropriate for shallow-to-moderate tissue, and a smooth profile under everyday clothing. These two consistently top-rated options are available across the 40-band A and B cup range and earn high marks for all-day comfort, band stability, and genuine fit accuracy at wider band sizes.
Glamorise Women’s MagicLift Active Support Wirefree Bra #1005
The standout wire-free everyday bra for A and B cup wearers on wider 40-inch bands who want all-day lift and comfort without underwire. At A and B cup on a 40 band, wire-free options often fail to separate and shape the tissue — the MagicLift internal sling system addresses this directly, lifting and separating without any wire and delivering a shaped, smooth silhouette that generic wire-free designs at these cup sizes do not achieve. The 40-band construction stays level across a broader ribcage without rolling or riding up — a specific challenge on wider bands where band tension must be distributed over more circumference. Wide, cushioned straps reduce shoulder pressure.
Moisture-wicking cotton-blend fabric handles full-day wear with comfort and breathability. Multi-hook back provides four levels of band tension adjustment — important on a 40 band where the band’s elastic relaxes measurably with regular wear. Range covers 30B through 46H, confirming purpose-built wider-band engineering.
Available in: 40A, 40B, and full range through 46H · Multiple colors
View on Amazon →
Glamorise Women’s WonderWire Front-Close Underwire Bra #1245
The underwire option purpose-built for wider band sizes at A and B cup volume — including 40A and 40B. The WonderWire cushioned underwire channel is the defining feature: it distributes wire pressure evenly across a wider, padded surface rather than concentrating force at the wire ends, reducing the side-poking and ribcage edge pressure that standard underwires cause on wider bands where the wire-to-body contact area is more pronounced. Front closure removes the behind-the-back fastening challenge entirely — a practical advantage that becomes more noticeable at 40-inch band sizes where closing a standard back-hook can require more arm extension.
Wide, flat side panels keep the band smooth under clothing without back bulge or roll. Multi-hook back provides three full levels of band adjustment to maintain correct tension as the fabric stretches with regular wear. 9,100+ Amazon reviews at 4.3 stars across the extended range through 48H — one of the most consistently reviewed underwire options in the wider-band, smaller-cup category.
Available in: 40A, 40B, and extended range through 48H · Multiple colors
View on Amazon →ℹ️ As an Amazon Associate, Bra Calculator earns from qualifying purchases. Product availability and pricing are subject to change. Always verify cup depth and sizing with the brand’s own chart before ordering — construction varies by style even within the same brand. For 40A specifically, confirm the brand stocks this combination before ordering, as some wider-band ranges begin at B cup.
5-Step Bra Fit Test: Confirm 40A or 40B Right Now
On the same 40 band, the choice between 40A and 40B is determined entirely by cup volume. These five checks confirm immediately whether your cup matches your breast tissue — and whether the 40 band itself is the correct band for your ribcage.
Lean forward and scoop all breast tissue — including any side fullness toward the armpits — fully forward and upward into the cup. Overflow at the top or sides after scooping means the cup is too small: move up one cup letter on the same 40 band (40A → 40B, or 40B → 40C). Wrinkling or gaping fabric after scooping means the cup is too large: move down one letter on the same band (40B → 40A, or 40A → consider wire-free styles with adjustable stretch cups). Do not switch bands to resolve a cup fit issue.
The centre gore must lie completely flat against the sternum throughout the day. A floating gore is almost always a sign the cup is too small. In a 40A, a persistently floating gore is the clearest signal you need 40B on the same band — go up one cup letter before making any other adjustment. This check takes priority over all other fit signals. If the gore lies flat but the band rides up, that is a band problem — the cup fit is correct but the band is too wide for your ribcage.
The underwire must encircle all breast tissue and rest entirely on firm ribcage. Because 40A and 40B share the same band, the underwire width across the chest wall is identical in both sizes — only the cup arc height differs. If the underwire sits on breast tissue at the top of the arc rather than beneath it, the cup arc is too low — try 40B. If the underwire extends visibly past your breast tissue onto bare ribcage on the sides, the band may be too wide for your chest width — consider sister size 38B (for 40A volume) or 38C (for 40B volume), which have narrower underwires built for a smaller chest wall.
On the loosest hook, slide two fingers under the back band — firm, consistent resistance is required. The band must run horizontally level all the way around the body. On a 40 band, this check also tells you whether the 40 band is correct for your ribcage overall. A 40 band sits correctly on approximately 35–36 inch underbust. If the band rides up in both 40A and 40B, neither cup size solves the underlying band problem — you need a tighter band via sister sizing: 38B (for A-cup volume) or 38C (for B-cup volume).
Raise arms overhead, twist side to side, and walk briskly for 60 seconds. The band must stay completely level, cups must hold all breast tissue in position, and straps must stay on the shoulders without slipping or digging. If cups shift downward or tissue escapes during movement, the cup is too small — try 40B if in 40A. If cups collapse or gape during movement, the cup is too large — try 40A if in 40B. Any downward drift of the cups during movement is a band problem, not a strap problem — tightening straps never permanently resolves a loose band.
Still unsure whether 40A or 40B — or a completely different size — is right for your measurements? Get a precise answer in seconds, including your sister size alternatives.
Try the AI-Powered Bra Size Calculator →
40A vs 40B: Full Side-by-Side Comparison
| Feature | 40A | 40B |
|---|---|---|
| Band Size | 40 — same band, ~35–36″ / 89–91 cm underbust | |
| Cup Letter (US/UK) | A — 1″ (2.5 cm) over underbust | B — 2″ (5 cm) over underbust |
| EU Equivalent | 90A | 90B |
| AU / NZ Equivalent | 22A | 22B |
| Fullest Bust | ~41″ (104 cm) | ~42″ (107 cm) |
| Cup Volume (per breast) | ~550 ml | ~700 ml (+150–175 ml over 40A) |
| Cup Depth | Shallow — minimal projection | Moderate — more forward depth |
| Sister Sizes? | ❌ No — same band, different volumes. Not sister sizes. | |
| Band Construction | Identical — same 40-inch frame, same underwire width, same hook-and-eye length | |
| Sister Size Family | 36C — 38B — 40A — 42AA | 36D — 38C — 40B — 42A — 44AA |
| If cups too small | Try 40B (same band, next cup up) | Try 40C (same band, next cup up) |
| If cups too large | Try wire-free/bralette styles with stretch cups | Try 40A (same band, prior cup) |
| If band too loose | Try 38B (sister — same volume, tighter band) | Try 38C (sister — same volume, tighter band) |
| If band too tight | Try 42AA (sister — same volume, wider band) | Try 42A (sister — same volume, wider band) |
| Best breast shape | Shallow, east-west, minimal projection | Rounder, slightly more projected, fuller |
| T-shirt silhouette | Flat to minimal projection — very discreet | Gentle curve — T-shirt friendly |
| Bralette compatibility | Excellent — most styles work at A cup | Excellent — most styles work at B cup |
| Availability | Limited — 40A often not stocked; 38B (sister) is more accessible | Moderately available — wider coverage in plus-size ranges |
| Practical alternative | 38B — same volume, more widely stocked | 38C — same volume, available at most retailers |
People Also Ask: 40A vs 40B — Answered
Are 40A and 40B the same size?
No — 40A and 40B are not the same size. They share the same 40-inch band but hold different cup volumes. A 40B contains one full cup size more breast tissue than a 40A — approximately 150–175 ml more per cup. The band fits the same ribcage; the cup depth and enclosed volume are genuinely different. These are not sister sizes.
Which is bigger — 40A or 40B?
40B is larger in cup volume. A 40A fits a 1-inch bust-to-underbust differential (approximately 41-inch bust) while a 40B fits a 2-inch differential (approximately 42-inch bust) — on the same 40-inch band. The 40B cup is deeper, projects more forward, and holds approximately 150–175 ml more breast tissue per cup than the 40A.
Are 40A and 40B sister sizes?
No. Sister sizes are sizes on different bands that hold equal cup volume. 40A and 40B are on the same band and hold different cup volumes — so they cannot be sister sizes. The sister size of 40A is 38B (tighter band, same volume). The sister size of 40B is 38C (tighter band, same volume). These are entirely separate volume families with no overlap.
My 40A cups overflow — should I try 40B?
Yes — if your 40A cups overflow, spill over the top, or the centre gore floats away from your sternum while the band fits correctly, 40B on the same band is the correct next step. The cup is the variable here, not the band. Do not switch to 38A or any other band-change to resolve a cup overflow problem — change the cup letter on the same 40 band.
My 40B cups gap and wrinkle — should I try 40A?
Yes — if your 40B cups gap, wrinkle, or collapse after the scoop-and-swoop check, and the band fits correctly, try 40A on the same band. Cup gapping means the cup is too large for your breast tissue volume. The move is 40B → 40A on the same band, not to a different band. If 40A also gaps slightly, consider bralette or soft-cup styles with stretch-cup construction that better adapts to minimal tissue volume.
What is the sister size of 40A?
The sister sizes of 40A are 38B (one band tighter, same cup volume) and 42AA (one band wider — where available). The most practical sister size for 40A is 38B, which is significantly more commonly stocked than 40A itself. On a 36-inch underbust, 36C is also a sister size of 40A. If your 40A band rides up, 38B gives the same cup volume with a firmer, better-anchored band that delivers noticeably better support.
What is the sister size of 40B?
The sister sizes of 40B are 38C (one band tighter, same cup volume), 42A (one band wider), and 36D (two bands tighter). If your 40B band consistently rides up during the day, 38C gives you the same cup volume with a firmer anchor — and roughly 80% of bra support comes from the band, so the improvement in lift and stability can be significant. If the 40B band is too tight, 42A provides the same volume with a wider, more relaxed fit.
Is 40A a common bra size?
40A is not widely stocked in mainstream retail. The combination of a wide 40-inch band with the smallest standard A cup means it fills a narrow market segment — many brands that carry wider bands start at B cup. The sister size 38B is considerably more accessible and holds the same cup volume. If you measure as a 40A and cannot find your size, 38B is the most practical starting point for the same cup volume with confirmed retail availability.
Is 40A considered plus size?
The “40” refers only to the band width — a 40-inch underbust — not to body size or dress size. A 40 band is a wider band that sits in the full-figure and plus-size section of many retailers’ ranges due to the band width, but cup-wise it is the smallest standard cup in the 40-band range. Many 40A wearers have broader ribcages with modest breast tissue — the combination of wide band and small cup is a legitimate body proportion, not an indicator of overall body size.
How do I know my correct bra size between 40A and 40B?
Measure your underbust snugly for band (approximately 35–36 inches = 40 band) and your fullest bust for cup. Subtract underbust from bust: 1 inch = A cup, 2 inches = B cup. Verify fit with five checks: level band, flat gore, underwire fully under breast tissue, two fingers under back band on loosest hook, no cup movement during activity. Use our bra size chart calculator for a precise personalised result including sister size alternatives.
