38A vs 38B Bra Size: What Is the Real Difference? (2026 Fit Guide)
Quick Answer: No — 38A and 38B are not the same size. They share the same band number (38) and therefore fit the same ribcage, but they hold different cup volumes. A 38B contains one full cup size more breast tissue than a 38A. The band is identical; the cup is the only variable — and that one letter difference is meaningful.
If you are deciding between 38A and 38B, the answer is straightforward: choose the size whose cup volume matches your breast tissue. Neither is “better” — they suit different bust volumes on the same 38-inch frame.
⚡ Key Takeaways
- Not the same size: 38A and 38B share one band but hold different cup volumes — 38B is one full cup larger.
- Same band, different cup: Both fit an underbust of ~33–34″ (84–86 cm). Only the bust measurement and cup depth differ.
- 38A cup: 1″ bust-to-underbust differential — shallower cup, less enclosed volume.
- 38B cup: 2″ bust-to-underbust differential — deeper cup, one full size more volume than 38A.
- Not sister sizes: Sister sizing requires a band change. 38A and 38B are on the same band — cup volume is simply different.
- Sister size of 38A: 36B (tighter band, same cup volume as 38A).
- Sister size of 38B: 36C (tighter band, same cup volume as 38B).
- Cup overflow in 38A → try 38B: Same band, more cup space — no band adjustment needed.
- Gaps or wrinkling in 38B → try 38A: Same band, less cup space — again, no band change required.
- UK/EU sizing aligns: US 38A = UK 38A = EU 85A; US 38B = UK 38B = EU 85B.
38A vs 38B: The Core Difference Explained
Both 38A and 38B are built around the same band number — meaning the horizontal band that wraps around your ribcage is engineered for the same underbust circumference of approximately 33–34 inches (84–86 cm). Band construction, hook-and-eye length, underwire span, and strap width are all virtually identical between the two sizes.
The only variable is the cup. Cup letters represent the difference between your fullest bust measurement and your underbust measurement — each inch equals one cup letter:
- 1″ difference = A cup
- 2″ difference = B cup
- 3″ difference = C cup
- 4″ difference = D cup
A 38A means your bust is 1 inch larger than your underbust on a 38-inch frame — a shallow cup with modest enclosed volume. A 38B means your bust is 2 inches larger than your underbust on that same frame — a notably deeper cup with meaningfully more space inside. The difference between them is one full cup size of breast tissue volume.
The Volume Difference Visualised
38B holds one full cup size more volume than 38A — same band, genuinely different cups
This is the core distinction that makes 38A vs 38B fundamentally different from a sister size comparison. When you switch from 38A to 38B, you are not preserving volume on a different frame — you are increasing the volume inside the cup while keeping the band identical. If your 38A cups overflow, the next logical step is 38B on the same band — not a sister size swap to a different band.
Measurement Breakdown: 38A vs 38B
The table below shows exactly how each size is derived from body measurements and what that means for the cup space each one provides.
| Size | Underbust (Band) | Bust (Fullest Point) | Cup Difference | Cup Volume |
|---|---|---|---|---|
| 38A | ~33–34″ (84–86 cm) | ~34–35″ (86–89 cm) | 1″ (2.5 cm) | Smaller ↓ |
| 38B | ~33–34″ (84–86 cm) | ~35–36″ (89–91 cm) | 2″ (5 cm) | Larger ↑ (one full cup more) |
EU sizing: US 38A = EU 85A; US 38B = EU 85B. UK sizing: Both align — US 38A = UK 38A; US 38B = UK 38B. Cup letters at A and B are consistent across US and UK systems, making international conversions straightforward at this size range. Always verify with the brand’s own chart before ordering, as construction varies by manufacturer. Use our international bra size charts for full reference.
Sister Sizes: 38A and 38B Each Have Their Own Ladder
Because 38A and 38B hold different cup volumes, they belong to different sister size families. Sister sizes are sizes on different bands that hold the same cup volume — and since 38A and 38B have different volumes, their sister size chains do not overlap.
Sister Size Family for 38A
All sizes below hold the same cup volume as 38A:
If your 38A band feels too tight, try 40AA (wider band, same cup volume — where available). If it feels too loose, try 36B (tighter band, same cup volume).
Sister Size Family for 38B
All sizes below hold the same cup volume as 38B:
If your 38B band feels too tight, try 40A (wider band, same cup volume). If it feels too loose, try 36C (tighter band, same cup volume). Explore complete chains on our sister sizes guide or generate your personal ladder with the sister size bra calculator.
The Same-Band Cup Rule Applied to 38A and 38B
| Comparison Type | Example | Same Volume? | What Changes |
|---|---|---|---|
| Same band, cup up | 38A → 38B | ❌ No | Cup volume increases by one full size — band stays the same |
| Same band, cup down | 38B → 38A | ❌ No | Cup volume decreases by one full size — band stays the same |
| Band down + Cup up (sister) | 38A → 36B | ✅ Yes | Tighter band, same cup volume as 38A |
| Band up + Cup down (sister) | 38A → 40AA | ✅ Yes | Wider band, same cup volume as 38A (where AA available) |
| Band down + Cup up (sister) | 38B → 36C | ✅ Yes | Tighter band, same cup volume as 38B |
| Band up + Cup down (sister) | 38B → 40A | ✅ Yes | Wider band, same cup volume as 38B |
| Same band, two cups up | 38A → 38C | ❌ No | Cup volume increases by two full sizes |
The rule is simple: changing only the cup letter on the same band always changes volume. Sister sizing only applies when you change both the band AND the cup letter simultaneously — one up, one down. Within the 38-band family, every step from A to B to C to D represents a genuine, measurable increase in cup volume.
Who Should Wear 38A?
- Underbust measurement approximately 33–34 inches (84–86 cm) snugly measured just beneath the bust.
- Bust measurement at the fullest point of approximately 34–35 inches (86–89 cm) — a 1-inch differential.
- Breast tissue is relatively modest in volume; cups fill naturally without overflow or lateral spillage.
- Current bra cups show no overflow, no floating gore, and no wrinkling or gaping fabric after the scoop-and-swoop check.
- In UK and EU sizing, shop for 38A / EU 85A — labels align across all three systems at this cup size.
- If 38A cups overflow or the gore floats: move up to 38B on the same band — do not change the band.
- If 38A band is too loose: move to 36B (same cup volume, tighter band) — not 38B.
Verify your fit with the five-point check on our how to know your bra fits page.
Who Should Wear 38B?
- Underbust measurement approximately 33–34 inches (84–86 cm) snugly measured just beneath the bust.
- Bust measurement at the fullest point of approximately 35–36 inches (89–91 cm) — a 2-inch differential.
- Breast tissue fills a B-cup space comfortably — cups contain all tissue without gaps, overflow, or a floating centre gore.
- Previously tried 38A and found the cups too small — overflowing, gaping gore, or underwire sitting on breast tissue rather than ribcage.
- In UK and EU sizing, shop for 38B / EU 85B — labels align consistently.
- If 38B cups overflow: move up to 38C on the same band — do not change the band.
- If 38B band is too loose: move to 36C (same cup volume, tighter band) — not 38A.
Use our breast shape identifier and size charts to confirm the best style for your shape.
🛍️ Best Bras for 38A and 38B — Our Top Picks
Women wearing a 38 band need bras with a well-anchored back band, correctly proportioned cups for modest-to-medium volume busts, and a comfortable fit that stays level through daily activity. These two top-rated options on Amazon are available in both 38A and 38B and consistently receive excellent reviews for comfort, band stability, and all-day wearability.
Glamorise Women’s MagicLift Active Support Wirefree Bra #1005
The top wire-free everyday bra for 38A and 38B wearers who want genuine lift and all-day comfort without an underwire. The MagicLift internal sling system lifts and separates naturally — giving A and B cup sizes the shape and separation that wire-free bras often fail to deliver. The 38-band construction sits level and stays anchored throughout the day. Wide cushioned straps eliminate shoulder strain, and the moisture-wicking cotton-blend fabric keeps you cool and comfortable across all activities. Multi-hook back gives full adjustability — critical for getting the band tension exactly right on a 38-inch frame.
Available in: 38A, 38B, and full range from 30B through 46H
View on Amazon →
Glamorise Women’s WonderWire Front-Close Underwire Bra #1245
An underwire bra built specifically for wider band sizes including 38A and 38B. The WonderWire cushioned underwire channel distributes pressure evenly — eliminating the side-poking and ribcage irritation that affects many A and B cup wearers in standard-gauge underwire bras. The front-close design makes everyday dressing faster and easier, while wide side panels keep the band flat and smooth under clothing. Multi-hook back provides three levels of band adjustment so you can fine-tune fit as the bra stretches over time. One of the most consistently praised underwire bras in the 38-band range on Amazon.
Available in: 38A, 38B, and extended range through 48H
View on Amazon →ℹ️ As an Amazon Associate, Bra Calculator earns from qualifying purchases. Product availability and pricing are subject to change. Always verify sizing with the brand’s own chart before ordering, as cup depth and band construction vary by style.
5-Step Bra Fit Test: Confirm 38A or 38B Right Now
On the same 38 band, the distinction between 38A and 38B comes down to one question: does the cup volume match your breast tissue? Run these five checks in whichever size you are currently wearing to find out immediately.
Lean forward and scoop all breast tissue — including any side fullness toward the armpits — fully forward and upward into the cup. Any overflow at the top or sides after scooping means the cup is too small: go up one cup letter on the same band (38A → 38B, or 38B → 38C). Wrinkling or gaping fabric means the cup is too large: go down one letter (38B → 38A).
The centre gore must lie completely flat against the sternum from morning to evening. A floating gore — even a small gap — almost always means the cup is too small. In a 38A, this is the most common signal that you need to move to 38B. Go up one cup letter on the same 38 band before making any other adjustment.
The underwire must encircle all breast tissue and rest entirely on firm ribcage at every point. Because 38A and 38B share the same band, the underwire length across the chest is nearly identical — the difference is in the cup depth above the wire. If the wire sits on breast tissue rather than beneath it, the cup is too small and you need the next cup size up on the same band.
On the loosest hook, slide two fingers under the back band — you must feel firm, consistent resistance. The band should run level all the way around. Because 38A and 38B have the same band, this check tells you whether the 38 band is correct for your ribcage overall. If the band rides up, both 38A and 38B will fail — and you need to move to 36B or 36C (sister sizes) depending on your cup volume.
Raise arms overhead, twist side to side, and walk briskly for 60 seconds. In a correctly fitting 38A or 38B, the band stays level, cups hold position, and straps stay on the shoulders without adjustment. If cups shift downward or tissue escapes during movement, the cup is too small — try 38B if you are in 38A. If cups gape or collapse, the cup is too large — try 38A if you are in 38B.
Not sure whether 38A, 38B, or a completely different size is the right fit for your measurements? Our AI calculator gives you a precise answer in seconds.
Try the AI-Powered Bra Size Calculator →
38A vs 38B: Full Side-by-Side Comparison
| Feature | 38A | 38B |
|---|---|---|
| Band Size | 38 — same band, ~33–34″ / 84–86 cm underbust | |
| Cup Letter (US/UK) | A — 1″ (2.5 cm) over underbust | B — 2″ (5 cm) over underbust |
| EU Equivalent | 85A | 85B |
| Cup Volume | Smaller | Larger — one full cup more than 38A |
| Sister Sizes? | ❌ No — same band, different volumes. Not sister sizes. | |
| Bust Measurement | ~34–35″ (86–89 cm) | ~35–36″ (89–91 cm) |
| Cup Depth | Shallower — 1-inch differential | Deeper — 2-inch differential |
| Band construction | Identical — same 38-inch frame, same hook-and-eye length | |
| Sister size family | 34C — 36B — 38A — 40AA | 34D — 36C — 38B — 40A |
| If cups too small | Try 38B (same band, next cup up) | Try 38C (same band, next cup up) |
| If cups too large | Try 38AA (same band, prior cup — if available) | Try 38A (same band, prior cup) |
| If band too tight | Try 40AA (sister size — same volume) | Try 40A (sister size — same volume) |
| If band too loose | Try 36B (sister size — same volume) | Try 36C (sister size — same volume) |
| Availability | Moderately available — less common than 38B in mainstream retail | Widely available — one of the most stocked sizes in the 38 band |
| Best For | ~1-inch bust-to-underbust differential on a 38-inch frame | ~2-inch bust-to-underbust differential on a 38-inch frame |
People Also Ask: 38A vs 38B — Answered
Are 38A and 38B the same size?
No — 38A and 38B are not the same size. They share the same 38-inch band but hold different cup volumes. A 38B contains one full cup size more breast tissue than a 38A. The band fits the same ribcage; the cup is genuinely different. These are not sister sizes — sister sizing requires changing both the band and the cup letter simultaneously.
Which is bigger — 38A or 38B?
38B is bigger in cup volume. A 38A has a 1-inch bust-to-underbust differential (A cup) and a shallower cup. A 38B has a 2-inch differential (B cup) and a noticeably deeper, larger cup. On the same 38-inch band, one additional inch of differential equals one full additional cup size of enclosed breast tissue.
Are 38A and 38B sister sizes?
No. Sister sizes are sizes on different bands that hold equal cup volume. 38A and 38B are on the same band and hold different cup volumes — so they are not sister sizes by definition. The sister size of 38A is 36B (tighter band, same volume). The sister size of 38B is 36C (tighter band, same volume). These are separate volume families.
My 38A cups overflow — should I try 38B or 36B?
If your 38A cups overflow but the band fits correctly, try 38B on the same band — not 36B. Moving to 36B would give you the same cup volume as 38A but in a tighter band. You need more cup space, not a different band. Go up one cup letter: 38A → 38B. If both your cups and band need to change, assess band fit separately first.
My 38B band feels too loose — should I try 36B or 38A?
If your 38B band feels too loose but the cups fit correctly, try 36C — not 36B or 38A. Moving to 36C gives you a tighter band while preserving the same cup volume as your 38B. Moving to 38A reduces cup volume without changing the band — that solves nothing. Moving to 36B gives you a tighter band but also less cup volume — correct only if your cups are also too large.
What is the sister size of 38A?
The primary sister sizes of 38A are 36B (one band size tighter, same cup volume) and 40AA (one band size wider, same cup volume — where available). If your 38A band rides up or feels loose, try 36B for the same cup volume in a firmer, better-anchored band. 40AA is the correct move if the 38A band pinches.
What is the sister size of 38B?
The sister sizes of 38B are 36C (one band size tighter, same cup volume) and 40A (one band size wider, same cup volume). If your 38B band is too loose, try 36C — same cup space with a firmer band. If the 38B band feels too tight, try 40A for the same cup volume with a more relaxed fit around the torso.
Is 38A a common bra size?
38A is less commonly stocked than 38B in mainstream retail. The 38 band is a wide size and the A cup is a small volume — a combination that many mid-market retailers carry in limited styles. Specialist lingerie brands and online retailers offer the best selection for 38A. If you cannot find your size, sister size 36B is widely available and carries identical cup volume in a tighter band.
Can I wear a 38B if I measured as a 38A?
Only if your 38A cups are too small. Measurements give you a starting point, not a guaranteed fit. If your 38A cups overflow, the gore floats, or underwire sits on breast tissue rather than beneath it, 38B on the same band is the correct next step. If your 38A fits well, there is no reason to go to 38B — doing so would give you more cup space than your breast tissue fills, resulting in gaps and wrinkling.
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 — 1 inch = A cup, 2 inches = B cup. Then verify fit with five checks: level band, flat gore, underwire on ribcage, two fingers under back band, no movement during activity. Use our bra size chart calculator for a precise personalised result.
