40B vs 42A Bra Size: Are They Sister Sizes? Cup Volume & Fit Guide (2026)
Quick Answer: Yes — 40B and 42A are direct sister sizes. They have identical cup volume (approximately 700 ml per breast), but different band widths. 40B has a 40-inch band with a B cup, while 42A has a 42-inch band with an A cup. The rule is simple: band up 2 inches, cup letter down 1. Swap between them when the cup fits perfectly but the band is too tight or too loose.
⚡ Key Takeaways
- Same cup volume: 40B and 42A hold approximately 700 ml per breast — identical capacity.
- Different bands: 40B has a 40-inch band; 42A has a 42-inch band (2 inches wider).
- Sister size rule: Band +2 inches = Cup letter −1 (40B → 42A preserves volume).
- Both fit bust ~42–43″: The bust measurement that fills these cups is similar; underbust differs.
- Choose 40B when: You need firmer band support, maximum lift, or have a ~40″ underbust.
- Choose 42A when: The 40B band digs in, you prefer looser bands, or have a ~42″ underbust.
- 42A is rare stock: 40B is widely available; 42A requires specialty retailers.
- Full sister ladder: 38C — 40B — 42A — 44AA (all same cup volume).
What Is a Sister Size? The Band-Cup Trade-Off
A sister size is a bra size with the same cup volume but a different band width. The rule is simple: when you increase the band by 2 inches, you decrease the cup letter by one to preserve the same internal volume. When you decrease the band by 2 inches, you increase the cup letter by one.
This works because cup letters are relative to band size. A “B cup” on a 40 band is a specific volume (2-inch differential on a 40-inch frame). An “A cup” on a 42 band is calculated as a 1-inch differential on a 42-inch frame — but because the frame is larger, the actual cup space equals the B cup on the smaller frame.
The Sister Size Formula
- Band up +2″ → Cup down −1 letter (preserves volume)
- Band down −2″ → Cup up +1 letter (preserves volume)
Applied to 40B and 42A:
- 40B → increase band to 42″ → decrease cup from B to A → 42A (same volume)
- 42A → decrease band to 40″ → increase cup from A to B → 40B (same volume)
Both hold the same cup volume. Only the band width differs: 40″ vs 42″. Same breast tissue, different ribcage fit.
Measurement Breakdown: 40B vs 42A
The table below shows how each size is derived from body measurements.
| Measurement | 40B | 42A |
|---|---|---|
| Underbust (Band) | ~39–40″ (99–102 cm) | ~41–42″ (104–107 cm) |
| Bust (Fullest Point) | ~42″ (106.7 cm) | ~43″ (109.2 cm) |
| Bust-to-Band Difference | 2″ (5 cm) = B cup | 1″ (2.5 cm) = A cup |
| Approx. Cup Volume | ~700 ml per side (identical) | |
| Band Tension | Snug — fits ~40″ ribcage | Looser — fits ~42″ ribcage |
| Sister Size Relationship | ✅ Direct sisters — same cup volume, different band | |
Why the bust measurements differ slightly: 40B fits a ~42″ bust on a 40″ ribcage. 42A fits a ~43″ bust on a 42″ ribcage. The cup volume is identical; the bust measurement differs because the ribcage circumference differs.
The Complete Sister Size Family: Same Cup, Different Band
All sizes below hold the same cup volume as 40B and 42A. They’re interchangeable in cup fit — only the band width changes.
All four sizes hold identical cup volume (~700 ml per side). Choose based on your underbust measurement.
How to Use This Ladder:
- If your 40B band is too tight: Move up to 42A — same cup, wider band.
- If your 40B band is too loose: Move down to 38C — same cup, tighter band.
- If your 42A band is too tight: Move up to 44AA — same cup, widest band.
- If your 42A band is too loose: Move down to 40B — same cup, tighter band.
When to Choose 40B vs 42A
Choose 40B When:
- Your underbust measurement is approximately 39–40 inches (99–102 cm) snugly measured.
- You need firmer band support for maximum lift and stability.
- You prefer the band to anchor firmly without shifting during activity.
- You have a narrower ribcage that doesn’t fill out a 42 band.
- Your 42A band rides up or feels too loose.
Choose 42A When:
- Your underbust measurement is approximately 41–42 inches (104–107 cm) snugly measured.
- Your 40B band digs in painfully, leaves deep red marks, or restricts breathing.
- You prefer a looser band feel for comfort (though support may be slightly reduced).
- You’re between band sizes and need more room.
- You find 42A in stock and need the cup volume of 40B.
Real Fit Differences: What Actually Changes
| Fit Aspect | 40B | 42A |
|---|---|---|
| Band Grip | Firmer — anchors to ~40″ ribcage | Looser — 2″ more circumference |
| Cup Fit | Identical — same internal volume | |
| Support Level | Higher — band carries more weight | Moderate — may shift if too loose |
| Rib Pressure | Higher if 40 is tight for your frame | Lower — more breathing room |
| Strap Load | Lower — band does the work | Slightly higher — looser band shifts weight |
| Retail Availability | Common — most full-figure retailers | Rare — requires specialty search |
The cup is identical. The underwire shape, cup depth, and internal volume are the same. Only the band length and the position of the hook-and-eye closure change. If you try both sizes and the cup fits differently, you may have a brand inconsistency rather than a true sister size relationship.
The Sister Size Rule Applied to 40B and 42A
| What You’re Changing | Example | Same Volume? | What Happens |
|---|---|---|---|
| Band up +2″, cup down −1 | 40B → 42A | ✅ Yes | Looser band, same cup volume — sister size |
| Band down −2″, cup up +1 | 42A → 40B | ✅ Yes | Tighter band, same cup volume — sister size |
| Band up +2″, cup down −1 | 42A → 44AA | ✅ Yes | Even looser band, same cup volume — sister size |
| Band down −2″, cup up +1 | 40B → 38C | ✅ Yes | Tighter band, same cup volume — sister size |
| Same band, cup up +1 | 40B → 40C | ❌ No | Same band, larger cup — different volume |
| Same band, cup down −1 | 40B → 40A | ❌ No | Same band, smaller cup — different volume |
The governing rule: volume is only preserved when you change both band and cup letter simultaneously in opposite directions. Changing only the band (40B → 42B) or only the cup (40B → 40C) changes the volume. The sister size relationship specifically requires the +2″/−1 or −2″/+1 adjustment.
Where to Find 40B and 42A
40B Availability
40B is widely available as a standard full-figure size. Most department stores, big-box retailers, and online marketplaces stock 40B. Brands like Bali, Playtex, Warner’s, Wacoal, and Glamorise all produce 40B in multiple styles.
42A Availability
42A is significantly harder to find than 40B. This is because it combines a wide band (42) with a small cup (A), a less common combination in mass-market bra production. Your best sources:
- Glamorise — Extended size range including 42A
- Playtex — Selected styles in 42A
- Online retailers — Amazon, Bare Necessities, HerRoom
- Specialty full-figure boutiques — May order specific sizes
🛍️ Best Bras for 40B and 42A — Our Top Picks
Since 40B and 42A share the same cup volume, these recommendations work for both sizes — just select your appropriate band size. Look for styles available in both 40 and 42 bands with B and A cups respectively.
Glamorise Women’s MagicLift Active Support Wirefree Bra #1005
A top-rated wire-free everyday bra available in both 40B and 42A. The MagicLift internal sling system provides gentle support and separation without underwire — ideal for B-cup volumes on wider bands. The multi-hook back accommodates both 40 and 42 band sizes, and the stretch fabric adapts to slight band variations. If you’re between 40B and 42A, this style’s flexibility makes it ideal for testing which band feels better on your ribcage.
Available in: Band sizes 30–46 including 40 and 42, cups B–K · Multiple colors
View on Amazon →
Glamorise Women’s WonderWire Front-Close Underwire Bra #1245
A structured underwire option available in 40B and extended sizes including 42A. The WonderWire cushioned underwire channel distributes pressure evenly, while the front closure makes it easier to fasten for those with limited shoulder mobility. The band runs true to size in both 40 and 42, making sister size swapping straightforward. If you need the cup volume of 40B but find the band tight, sizing up to 42A in this style provides the same cup fit with relaxed band tension.
Available in: Full range through 48H including 40B and 42A equivalent · Multiple colors
View on Amazon →ℹ️ As an Amazon Associate, Bra Calculator earns from qualifying purchases. Product availability and pricing are subject to change. When shopping for sister sizes 40B and 42A, verify that the brand carries both sizes — many brands stop at 40B and don’t produce 42A. Glamorise and Playtex are reliable sources for both sizes.
5-Step Fit Test: Confirming 40B vs 42A Is Right for You
Because 40B and 42A share the same cup volume, the fit test focuses on band tension — the only difference between them. These five checks confirm which band size is right for your ribcage.
On the loosest hook, slide two fingers under the back band. If you feel firm resistance and the band stays level, the size is correct. If you can pull the band 3+ inches from your back or it rides up during wear, it’s too loose (try 40B instead of 42A). If you can’t slide two fingers under or the band leaves deep marks, it’s too tight (try 42A instead of 40B).
Scoop all breast tissue into the cup. The cup should contain all tissue without overflow at the top or sides. If the cup fits perfectly in 40B but the band is wrong, 42A will fit the same cup-wise. If the cup is wrong (spillage or gapping), don’t change to 42A — adjust the cup letter (40A or 40C) or try 38C/44AA for sister sizes.
If your 40B band is too tight, try 42A. The cup should fit identically; only the band tension should change. If the 42A band feels significantly better but the cup suddenly seems too large, you may need to stay with 40B and use a bra extender instead.
The band should run horizontally level all the way around your torso. If it rides up at the back in 42A, the band is too loose — you need the firmer grip of 40B (or possibly 38C). If it digs in at the front or sides in 40B, you need the looser fit of 42A.
Raise your arms overhead, twist side to side, and walk briskly for 60 seconds. The band should stay completely level in both 40B and 42A. If 40B restricts breathing during movement, 42A is the better choice. If 42A shifts or rides up during movement, 40B provides better support.
Not sure whether 40B, 42A, or a different sister size is right for your measurements? Our AI calculator gives you a precise answer — including your complete sister size ladder.
Try the AI-Powered Bra Size Calculator →
40B vs 42A: Full Side-by-Side Comparison
| Feature | 40B | 42A |
|---|---|---|
| Band Size | ~39–40″ / 99–102 cm | ~41–42″ / 104–107 cm |
| Bust Measurement | ~42″ / 106.7 cm | ~43″ / 109.2 cm |
| Bust-to-Band Difference | 2″ (5 cm) = B cup | 1″ (2.5 cm) = A cup |
| Cup Volume | ~700 ml per side — identical | |
| Sister Size Relationship | ✅ Direct sisters — same cup volume, different band | |
| Full Sister Ladder | 38C — 40B — 42A — 44AA (all same volume) | |
| Band Tension | Firmer — better support | Looser — more comfort |
| Best For | ~40″ underbust, firm support preference | ~42″ underbust, loose band preference |
| If Band Too Tight | Try 42A (sister — same cup, looser band) | Try 44AA (sister — same cup, looser band) |
| If Band Too Loose | Try 38C (sister — same cup, tighter band) | Try 40B (sister — same cup, tighter band) |
| Availability | Common — most retailers | Rare — specialty/full-figure brands |
People Also Ask: 40B vs 42A — Answered
Are 40B and 42A sister sizes?
Yes — 40B and 42A are direct sister sizes. They share identical cup volume (approximately 700 ml per breast) but have different band widths. The 42A band is 2 inches wider than 40B. If your 40B band feels too tight but the cup fits perfectly, 42A is the correct sister size to try.
Is 42A the same as 40B?
Same cup volume, different band. 40B and 42A hold approximately the same breast tissue volume but fit different ribcages. 40B is designed for a ~40-inch underbust; 42A is designed for a ~42-inch underbust. The cups are interchangeable in volume; only the band tension differs.
Should I size up from 40B to 42A?
Only if your 40B band is genuinely too tight. If the 40B band digs in, leaves deep red marks, or restricts breathing, try 42A for the same cup volume with a looser band. However, if the 40B band fits well but the cup is wrong (spillage or gapping), adjust the cup letter (40A or 40C) rather than changing to 42A.
Is 42A harder to find than 40B?
Yes — 42A is significantly harder to find than 40B. Most mainstream retailers stock 40B as a standard full-figure size, but 42A is rare because it combines a wide band (42) with a small cup (A). Online specialty retailers and full-figure brands like Glamorise are more likely to carry 42A.
Can I wear 42A if I usually wear 40B?
Yes — the cups will fit identically. You’ll get the same breast tissue coverage with a slightly looser band. Ensure the 42 band doesn’t ride up your back during movement. If it stays level, 42A is a valid alternative to 40B for comfort.
What’s the difference in cup size between 40B and 42A?
There is no difference in actual cup volume. The cup letter differs (B vs A) because cup letters are relative to band size. A B cup on a 40 band (2-inch differential) holds the same volume as an A cup on a 42 band (1-inch differential). Think of it as the same cup space labeled differently based on the frame size.
Which has better support — 40B or 42A?
40B offers firmer band support because the 40-inch band grips the ribcage more tightly. Approximately 80% of a bra’s support comes from the band, so a firmer band (40B) provides more lift and stability than a looser band (42A), assuming both fit correctly. 42A is better for comfort if the 40 band feels restrictive.
What are other sister sizes of 40B?
The full sister size family for 40B/42A cup volume includes: 38C (one band tighter, same cup volume), 40B (reference size), 42A (one band looser, same cup volume), and 44AA (two bands looser, same cup volume). All four sizes hold identical breast tissue volume on different band widths.
Should I buy both 40B and 42A?
If you’re between band sizes or find that different brands run tight or loose, having both can be useful. Some people prefer 40B for active days (firmer support) and 42A for relaxed days (comfort). However, since 42A is harder to find, you might use a bra extender on 40B to achieve 42A band sizing instead.
