40C vs 42B: Are They Sister Sizes? Same Cup Volume, Different Band Explained (2026)
Quick Answer: Yes — 40C and 42B are confirmed sister sizes. They hold approximately the same cup volume on different band sizes. Going up one band from 40 to 42 and dropping one cup letter from C to B follows the sister size rule exactly, preserving cup volume across the two sizes.
The difference between them is entirely in the band: a 40C grips a narrower ribcage more firmly and produces a slightly deeper, more projected cup, while a 42B sits on a wider ribcage with a broader, slightly shallower cup. Neither size is “larger” — your underbust measurement decides which fits correctly.
⚡ Key Takeaways
- Sister sizes: 40C and 42B hold approximately equal cup volume (~850 ml per breast) — same volume family.
- Band is the only real difference: 40C fits ~33–34″ (84–87 cm) underbust; 42B fits ~35–36″ (89–91 cm) underbust.
- Sister size rule: Up one band + down one cup letter = same volume preserved.
- UK/EU alignment: US 40C = UK 40C = EU 90C; US 42B = UK 42B = EU 95B. No naming divergence at C and B cup.
- Full sister size family: 36DD — 38D — 40C — 42B — 44A.
- Correct substitution: If 40C is unavailable, 42B is the right swap — not 42C.
- 40C is more widely stocked than 42B in mainstream retail.
- If 40C band digs in: try 42B — same cup volume, wider band, no cup adjustment needed.
- If 42B band rides up: try 40C — same cup volume, firmer band, better lift and stability.
- If cups are wrong in either size: change cup letter on the same band first, not the band.
Why 40C and 42B Hold the Same Cup Volume
Cup letters in bra sizing are ratios, not fixed measurements. Each letter represents the number of inches by which your fullest bust exceeds your underbust. When the band increases by one size — adding approximately 2 inches of circumference to the frame — dropping one cup letter offsets that increase precisely, keeping the total enclosed volume constant.
- 1″ difference = A | 2″ = B | 3″ = C | 4″ = D | 5″ = DD
A 40C means the bust is 3 inches larger than the underbust on a 40-inch frame. A 42B means the bust is 2 inches larger on a 42-inch frame. The wider base of the 42 frame compensates for the smaller cup differential — producing a cup space that contains approximately the same breast tissue volume as the 40C. A B cup on a 42 band holds the same tissue as a C cup on a 40 band.
The Volume Equivalence Visualised
≈ Equal cup volume (~850 ml per breast) — confirmed sister sizes
The critical distinction: this is not the same as comparing 40C to 42C. Moving to 42C while keeping the C letter would add one full cup size of volume on top of the wider band — a significantly larger bra. 42B is the correct sister of 40C — not 42C. See our cup size visuals page for a detailed breakdown of how volume changes across band and cup combinations.
Measurement Breakdown: 40C vs 42B
| Measurement | 40C | 42B |
|---|---|---|
| Underbust (Band) | ~33–34″ (84–87 cm) | ~35–36″ (89–91 cm) |
| Fullest Bust | ~43″ (109 cm) | ~44″ (112 cm) |
| Cup Differential | 3″ (8 cm) — C cup | 2″ (5 cm) — B cup |
| Cup Volume (per breast) | ≈ 850 ml ↔ | ≈ 850 ml ↔ |
| UK Equivalent | 40C | 42B |
| EU Equivalent | 90C | 95B |
| AU / NZ Equivalent | 22C | 24B |
The bust measurements differ slightly between the two — this is entirely normal for sister sizes. The 42B cup compensates for its smaller letter differential with a wider circumference, spanning more chest surface area. The result is approximately equal enclosed volume despite the different numbers and letters on the label.
The Sister Size Family: Where 40C and 42B Both Belong
40C and 42B 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 remains constant throughout the full family.
40C / 42B Sister Size Family
Every size in this chain is a valid substitute for 40C or 42B in terms of cup volume. The most practical sister size swaps are 38D (one band tighter than 40C, same volume — delivers a firmer anchor and stronger lift) and 44A (one band wider than 42B, same volume). Moving more than one step from your correct band size is rarely a comfortable long-term solution — the band anchors poorly even if the cup fills correctly. Explore the full chain on our sister sizes guide or use the sister size bra calculator for your complete personal ladder.
Who Should Choose 40C, and Who Should Choose 42B?
Choose 40C if:
- Underbust measures ~33–34″ (84–87 cm) snugly
- 42B band rides up the back by midday or during activity
- You want maximum band support and lift stability
- 42B band feels loose, even on the tightest hook
- Breast tissue sits closer together with more forward projection
- You want the widest style selection — 40C is more stocked than 42B
Choose 42B if:
- Underbust measures ~35–36″ (89–91 cm) snugly
- 40C band leaves red marks or restricts breathing from first wear
- 40C band digs under the arms or across the torso by midday
- Breast tissue has a wider root or spreads more laterally
- 40C is out of stock in a style you want — 42B is the correct swap
- You prefer a more relaxed band feel without sacrificing cup volume
Band Fit: Why the 40 vs 42 Decision Matters
At C and B cup volume on wider 40 and 42 bands, the breast tissue weight is moderate but meaningful. Roughly 80% of a bra’s support comes from the band. A 40C band on a correctly measured 33–34 inch ribcage stays level all day, providing consistent lift. A 42B on the same ribcage will ride upward at the back within hours, shifting breast weight to the shoulder straps — causing the progressive neck and shoulder tension that most wearers accept as normal but is entirely avoidable with the correct band size.
Conversely, a 40C on a 35–36 inch ribcage will feel uncomfortably tight from the first wear, potentially causing red marks and shallow breathing by midday. The 42B was built for that wider frame — on the correct ribcage, it anchors equally well and performs equally to the 40C. Both sizes perform excellently on their correct body. Neither performs well on the other’s frame.
The Sister Size Rule Applied to 40C and 42B
| Comparison Type | Example | Same Volume? | Relationship |
|---|---|---|---|
| Band up + Cup letter down | 40C → 42B | ✅ Yes | Sister sizes — equal volume |
| Band up, same cup letter | 40C → 42C | ❌ No | 42C is larger — one full cup above 40C |
| Band down + Cup letter up | 40C → 38D | ✅ Yes | Sister sizes — equal volume, firmer band |
| Band up + Cup letter up | 40C → 42C | ❌ No | 42C is larger — not a sister size swap |
| Same band, cup up | 40C → 40D | ❌ No | 40D is one full cup larger on the same band |
| Same band, cup down | 40C → 40B | ❌ No | 40B is one full cup smaller on the same band |
| Band up + Cup down (further) | 42B → 44A | ✅ Yes | Sister sizes — equal volume, wider band |
| Band down + Cup up (further) | 40C → 36DD | ✅ Yes | Sister sizes — equal volume, much firmer band |
The governing rule: the only correct band-change substitute for 40C is 42B — up one band, down one cup letter. Moving to 42C adds both a wider band and more cup volume simultaneously — that is never a sister size swap. When 40C is unavailable, 42B gives the closest possible fit. When 42B is unavailable, 40C is the correct alternative. Any other move changes cup volume.
🛍️ Best Bras for 40C and 42B — Our Top Picks
At C and B cup on 40 and 42 bands, the priority is a band that stays level on a wider frame, correctly proportioned cups for moderate volume, and construction that keeps the band smooth under clothing. These two consistently top-rated options cover the two most important use cases — wire-free all-day comfort and structured underwire support — and are available across both 40C and 42B.
Glamorise Women’s MagicLift Active Support Wirefree Bra #1005
The standout wire-free bra for C and B cup wearers on 40 and 42 bands who need genuine all-day lift and support without underwire. The MagicLift internal sling system lifts and separates without any wire — delivering meaningful shape and cup separation at C and B cup volume where many generic wire-free designs on wider bands produce a flat, unsupported silhouette.
The 40 and 42 band constructions both anchor solidly on their respective ribcage sizes, with the multi-hook back providing precise band tension adjustment across three closure rows. This is especially valuable on wider bands where even one hook of difference changes all-day comfort noticeably. Wide, cushioned straps distribute moderate cup load evenly without shoulder dig. Moisture-wicking cotton-blend fabric manages daily heat and movement. Because 40C and 42B are sister sizes holding identical cup volume, this bra performs identically at both sizes — cup containment, lift, and shape are equal. Simply order the size matching your underbust measurement.
Available in: 40C, 42B, and full range 30B–46H · Multiple colors
View on Amazon →
Glamorise Women’s WonderWire Front-Close Underwire Bra #1245
An underwire bra engineered for wider band sizes at C and B cup volume — specifically 40C and 42B. The WonderWire cushioned channel distributes underwire pressure evenly across a padded surface rather than concentrating force at two points. On wider 40 and 42 bands, this matters significantly — the wire spans a broader chest wall than on narrower bands, and a cushioned channel eliminates the side-poking and ribcage irritation that standard-gauge underwires cause at these band sizes.
Because 40C and 42B are sister sizes, the cup construction performs identically in both — cup containment, depth, and shape are equal. Front closure removes the behind-the-back fastening challenge — a practical benefit on wider-band bras where back-hook closure can be harder to reach and fasten. Reinforced side panels keep the band flat and smooth without roll or dig. Multi-hook back delivers three full levels of tension adjustment. 9,100+ Amazon reviews at 4.3 stars confirms its reliability across the extended range through 48H.
Available in: 40C, 42B, 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. Because 40C and 42B are sister sizes with equal cup volume, cup performance is identical at both sizes — order whichever matches your underbust measurement. Always verify band sizing with the brand’s own chart before ordering.
5-Step Bra Fit Test: Confirm 40C or 42B Right Now
Because 40C and 42B hold the same cup volume, these checks are focused on identifying whether the band is the issue rather than the cup. Cup-related fit failures resolve by changing cup letter on the same band; band-related failures resolve by switching to the sister size on a different band.
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 band (40C → 40D, or 42B → 42C). Wrinkling or gaping fabric means the cup is too large: move down one letter on the same band (40C → 40B, or 42B → 42A). Do not switch between 40C and 42B to resolve a cup volume issue — they hold the same volume and the problem will persist.
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 — move up one cup letter on the same band before any other adjustment. If the gore lies flat but the band consistently rides up, that is when switching between 40C and 42B makes sense: 42B → 40C (tighter band, same cup volume). A floating gore is a cup signal; a rising band is a band signal. Address each independently.
The underwire must encircle all breast tissue and rest entirely on firm ribcage. Check wire width against your breast root: the wire should begin and end precisely at the outer edge of your breast tissue. On wider 40 and 42 bands, the underwire spans a broader chest wall than on narrower bands. Wire extending past your breast root onto bare ribcage = underwire too wide for your breast root width — try a different brand’s 40C or 42B with a narrower wire channel before changing size. Wire pressing into side breast tissue = cup arc too small, try the next cup up on the same band.
On the loosest hook, slide two fingers under the back band — firm, consistent resistance required. The band must run horizontally level all the way around. Roughly 80% of your bra’s support comes from this band. If the 40C band leaves marks, restricts breathing, or digs under the arms → try 42B (same cup, wider band). If the 42B band rides up at the back during normal activity → try 40C (same cup, firmer band). This single check is the entire practical decision point between these two sister sizes.
Raise arms overhead, twist side to side, and walk briskly for 60 seconds. In a correctly fitting 40C or 42B, the band stays completely level, cups hold all breast tissue in position, and straps stay on the shoulders without slipping or digging. Any band rising at the back during movement confirms the band is too wide — switch to 40C if you are in 42B. Any band digging or leaving marks confirms the band is too narrow — switch to 42B if you are in 40C. Cup position should be identical in both sizes when correctly fitted on the right ribcage.
Still unsure whether 40C, 42B, or a completely different size is right for your measurements? Get a precise answer — including your full sister size ladder — in under 60 seconds.
Try the AI-Powered Bra Size Calculator →
40C vs 42B: Full Side-by-Side Comparison
| Feature | 40C | 42B |
|---|---|---|
| Band Size | 40 — ~33–34″ / 84–87 cm underbust | 42 — ~35–36″ / 89–91 cm underbust |
| Cup Letter (US/UK) | C — 3″ (8 cm) over underbust | B — 2″ (5 cm) over underbust |
| EU Equivalent | 90C | 95B |
| AU / NZ Equivalent | 22C | 24B |
| Cup Volume (per breast) | ≈ 850 ml ↔ | ≈ 850 ml ↔ |
| Sister Sizes? | ✅ Yes — confirmed sister sizes, same volume family | |
| Fullest Bust | ~43″ (109 cm) | ~44″ (112 cm) |
| Cup Shape | Slightly deeper, more projected | Slightly broader, shallower |
| Underwire Width | Narrower — suits closer-set breast root | Wider — suits broader breast root |
| Band Tightness | Firmer — more vertical lift and stability | Wider — more relaxed, less rib pressure |
| Full Sister Family | 36DD — 38D — 40C — 42B — 44A | |
| If cups too small | Try 40D (same band, next cup up) | Try 42C (same band, next cup up) |
| If cups too large | Try 40B (same band, prior cup) | Try 42A (same band, prior cup) |
| If band too tight | Try 42B (sister — same volume, wider band) | Try 44A (sister — same volume, wider band) |
| If band too loose | Try 38D (sister — same volume, firmer band) | Try 40C (sister — same volume, firmer band) |
| Shoulder strap load | Lower — firmer band carries more weight | Slightly higher — wider band shares more to straps |
| Availability | More widely stocked — most mainstream retailers | Less common — specialist and full-figure retailers best |
| Best For | Underbust ~33–34″, projected or close-set breast shape | Underbust ~35–36″, wider-root or side-full breast shape |
People Also Ask: 40C vs 42B — Answered
Are 40C and 42B the same size?
40C and 42B are confirmed sister sizes — they hold approximately the same cup volume (~850 ml per breast) on different band sizes. The band is wider in 42B and the cup letter is smaller, but these two changes cancel each other out in terms of enclosed volume. They are not identical bras — band tension and underwire width differ meaningfully. The correct size depends entirely on your underbust measurement.
Are 40C and 42B sister sizes?
Yes — 40C and 42B are confirmed sister sizes. Moving up one band from 40 to 42 and dropping one cup letter from C to B follows the sister size rule exactly. They share the same sister size family: 36DD, 38D, 40C, 42B, 44A. All sizes in that chain hold approximately equal cup volume.
Which is bigger — 40C or 42B?
Neither holds more cup volume — they are sister sizes with approximately equal breast tissue capacity (~850 ml per breast). The 42B bra is physically wider due to its larger band, but the enclosed cup space is equivalent. On a narrower frame, the 40C cup appears slightly more projected; on a wider frame, the 42B distributes the same volume more broadly. Volume is equal; the correct fit depends on your ribcage measurement.
Can I substitute 42B for 40C?
Yes — 42B is the correct sister size substitute when 40C is unavailable. Cup volume will be approximately equal. The 42B band is 2 inches wider, which means it will feel noticeably looser on a 33–34 inch ribcage. If the looser band causes the bra to ride up your back during activity, return to 40C — the band fit is not correct on your frame regardless of cup volume match.
My 40C band feels too tight — should I try 42B or 42C?
If your 40C band is too tight but the cups fit correctly, try 42B — not 42C. The 42B is the sister size: it gives you a wider band while preserving exactly the same cup volume. Moving to 42C gives you a wider band AND more cup volume — correct only if both your band and cups are too small. Change one variable at a time.
My 42B cups feel too small — should I try 42C or 40C?
If your 42B cups feel too small but the band fits correctly, try 42C on the same band — not 40C. The 40C is the sister size of 42B, meaning it holds the same cup volume, not more. Moving to 40C would give you a tighter band with no additional cup space — solving neither problem. Always change one variable at a time.
Which gives better support — 40C or 42B?
For a ribcage measuring 33–34 inches, the 40C delivers better support because the firmer band anchors more securely. Roughly 80% of a bra’s support comes from the band. A correctly fitted 40C band stays level all day, while a 42B on the same ribcage rides upward and shifts the load to the straps. For a 35–36 inch ribcage, the 42B performs equally well — the 40C would be uncomfortably tight on that wider frame.
Is 42B harder to find than 40C?
Yes — 40C is more commonly stocked in mainstream retail. The 40 band is among the most widely produced, and C cup is a common cup letter. 42B is available but requires more searching — full-figure retailers, specialty lingerie stores, and online retailers offer the widest selection. When 42B is unavailable in a style you want, 40C is the correct sister size substitute providing the same cup volume in a firmer band.
Does band size change cup shape between 40C and 42B?
Yes — the 40C cup is built with a slightly narrower underwire channel and more forward depth, suited to a narrower 40-inch chest frame. The 42B cup has a wider underwire and a slightly broader, shallower profile built for a 42-inch frame. Same volume, different geometry — which is why underwire width is an important fit consideration beyond just volume when choosing between these two sizes.
How do I know my correct bra size at 40C or 42B?
Measure your underbust snugly for band: 33–34 inches = 40 band; 35–36 inches = 42 band. Measure fullest bust and subtract underbust: 3 inches = C cup; 2 inches = B cup. Verify fit with five checks: level band, flat gore, underwire fully beneath 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 your full sister size ladder.
