40A vs 40B
One cup letter apart on the same band. Here is what one inch of difference actually changes, and how to know which is yours.
40A or 40B, in short
40A and 40B share the 40 band; 40B holds one cup step more volume: about one inch of bust. You are 40B if 40A spills, floats at the gore or puts the wire on tissue; you are 40A if 40B gapes or wrinkles at the top. If the band is wrong in either, that is a sister-size change, not a cup change.
40A and 40B, side by side
Same band bar, bigger dome: one inch more bust on the same frame: the cup grows, the band does not.
40A or 40B?
- 40A cups spill over the top or at the sides
- The gore floats off the sternum in a 40A
- The wire sits on breast tissue in a 40A
- 40B cups gape or wrinkle at the top
- 40B molded cups leave a hollow at the apex
- 40A fills evenly with a smooth top edge
Both share the 40 band. If the band is wrong in either, that is a separate fix: sister sizes (38B / —), not a cup change.
Brands that carry both 40A and 40B
Range-checked against each brand's core collections, with both sizes written in the brand's own notation.
| Brand | 40A as | 40B as | System | How it runs |
|---|---|---|---|---|
| Wacoal | 40A | 40B | US | Band true to size · cups true |
| Natori | 40A | 40B | US | Band soft / stretchy · cups grades small |
| Le Mystère | 40A | 40B | US | Band firm · cups true |
| Victoria's Secret | 40A | 40B | US | Band soft / stretchy · cups grades small |
| Maidenform | 40A | 40B | US | Band true to size · cups true |
| Soma | 40A | 40B | US | Band true to size · cups true |
| ThirdLove | 40A | 40B | US | Band true to size · cups grades generous |
| Savage X Fenty | 40A | 40B | US | Band soft / stretchy · cups true |
| Aerie | 40A | 40B | US | Band true to size · cups true |
| Cuup | 40A | 40B | US | Band true to size · cups true |
Every brand page converts your measured size and notes how it runs. All bra brands →
40A and 40B in every system
| US | UK | EU / DE / JP | FR / ES | AU / NZ | IT |
|---|---|---|---|---|---|
| 40A | 40A | 90A | 105A | 18A | 6A |
| 40B | 40B | 90B | 105B | 18B | 6B |
The one-cup step is one letter in every alphabet, but above D the letters diverge (US DDD = UK E = EU F).
The five fit checks, 40A vs 40B
| Check | What right looks like | What it tells you here |
|---|---|---|
| Band | Level, snug on the loosest hook, two fingers underneath, no ride-up | Identical band in both: if it fails, change the band with a sister size, not the cup |
| Cups | Filled to the top edge with no gap and no spill | 40B fills where 40A spills; 40A fills where 40B gapes |
| Gore | Flat against the sternum | A floating gore in 40A points to 40B |
| Straps | Light duty, no digging | Unrelated to the cup step; a strap problem is a band problem |
| Wire | On the ribcage, not on tissue | Wire on tissue in 40A points to 40B |
What goes wrong choosing between 40A and 40B
| Mistake | Why it fails |
|---|---|
| Going up a band instead of a cup when 40A spills | A bigger band with the same letter is a bigger cup AND a looser band; 40B keeps the band and adds only volume |
| Sizing down to 40A for a 'lifted' look | Spillage isn't lift; it is a cup too small |
| Judging by a molded cup only | Molded cups exaggerate both gap and spill; check a seamed cup before deciding |
Availability, shape and the +4 check
Related comparisons
40A vs 40B: answered
What is the difference between 40A and 40B?
One inch of bust on the same 40″ band: 40A is a 1″ difference, 40B a 2″ difference. 40B holds one cup step more volume; the band is identical.
Is 40A bigger than 40B?
40B is bigger by one cup step (about an inch of bust). Same band.
Can I wear 40B instead of 40A?
Only if the 40A cups are actually too small: spillage, floating gore or wire on tissue. Going up a cup for a band problem makes both worse.
What are 40A and 40B in UK and EU sizing?
40A: UK 40A, EU 90A, FR 105A. 40B: UK 40B, EU 90B, FR 105B.
Do 40A and 40B fit the same in every brand?
The relationship holds everywhere; the fit doesn't. Brands with soft bands push you toward the snugger sister, firm bands toward the looser; brands whose cups grade small push you up a cup. The brand table above shows how each runs.
Which is more common, 40A or 40B?
40A is less common in mainstream stores; 40B is less common in mainstream stores.
Should I own both 40A and 40B?
Usually one is right and the other is a mistake in the cup, but shape-dependent: a molded style may fit in one and a seamed style in the other.
How do I measure to tell 40A from 40B?
Underbust snug (38.5–40.4″ gives band 40), bust at the fullest point; the difference sets the cup. Then confirm on the body with the five fit checks.
HauteFlair labels cups in UK letters, so these links already convert your size by position (a US DDD lands on a UK E, a US G on a UK F). HauteFlair is owned by the same company as Brasize.ai; that relationship never changes the sizing math, charts, or fit advice on this page. How this site makes money.
Not sure which you are?
Two measurements settle it, then read the guide for your size.
Calculate My Bra Size →