38B vs 40A
Same cup volume, different band: sister sizes. Here is exactly what changes, and when to move between them.
38B or 40A, in short
38B and 40A are sister sizes: the same cup volume on a band two inches longer. Move to 40A if the 38 band digs in; move to 38B if the 40 band rides up. The cups themselves don't change.
38B and 40A, side by side
Same dome, different band bar: the cups hold the same volume; the band is two inches longer.
From 38B to 40A, or back
- The 38 band digs in or leaves marks by evening
- You are on the loosest hook and it still feels tight
- The cups fit perfectly, no gape, no spill
- The 40 band rides up at the back
- You fasten it on the tightest hook from new
- Straps are carrying the weight
Sister sizes share volume, not shape. The wire on 38 is narrower and more projected; on 40 wider and shallower. Narrow roots usually prefer the snugger sister; wide roots the looser one.
Brands that carry both 38B and 40A
Range-checked against each brand's core collections, with both sizes written in the brand's own notation.
| Brand | 38B as | 40A as | System | How it runs |
|---|---|---|---|---|
| Wacoal | 38B | 40A | US | Band true to size · cups true |
| Natori | 38B | 40A | US | Band soft / stretchy · cups grades small |
| Le Mystère | 38B | 40A | US | Band firm · cups true |
| Victoria's Secret | 38B | 40A | US | Band soft / stretchy · cups grades small |
| Maidenform | 38B | 40A | US | Band true to size · cups true |
| Soma | 38B | 40A | US | Band true to size · cups true |
| ThirdLove | 38B | 40A | US | Band true to size · cups grades generous |
| Savage X Fenty | 38B | 40A | US | Band soft / stretchy · cups true |
| Aerie | 38B | 40A | US | Band true to size · cups true |
| Cuup | 38B | 40A | US | Band true to size · cups true |
Every brand page converts your measured size and notes how it runs. All bra brands →
38B and 40A in every system
| US | UK | EU / DE / JP | FR / ES | AU / NZ | IT |
|---|---|---|---|---|---|
| 38B | 38B | 85B | 100B | 16B | 5B |
| 40A | 40A | 90A | 105A | 18A | 6A |
Sister sizes stay sisters in every system: the band moves one step and the cup one letter, whatever the alphabet.
The five fit checks, 38B vs 40A
| Check | What right looks like | What it tells you here |
|---|---|---|
| Band | Level, snug on the loosest hook, two fingers underneath, no ride-up | Both bands should pass this; the one that rides up or digs is the wrong sister |
| Cups | Filled to the top edge with no gap and no spill | Should look identical in both: the volume is the same |
| Gore | Flat against the sternum | Same in both |
| Straps | Light duty, no digging | Digging in the 40 band means the band is loose: take the snugger sister |
| Wire | On the ribcage, not on tissue | Narrower on 38, wider on 40: pick by breast root |
What goes wrong choosing between 38B and 40A
| Mistake | Why it fails |
|---|---|
| Going to 40A because the cups spill | Sister sizes hold the same volume: spillage needs a cup up on the same band, not the looser sister |
| Choosing the looser sister for comfort | A band that is comfortable on day one is loose by month three; the snugger sister on the loosest hook is the durable choice |
| Ignoring wire width | Same volume, different wire: narrow roots dig in the wider sister, wide roots pinch in the narrower one |
Availability, shape and the +4 check
Related comparisons
38B vs 40A: answered
What is the difference between 38B and 40A?
They are sister sizes: the same cup volume on a band two inches longer. 38B is a 38″ band with a 2″ difference; 40A is a 40″ band with a 1″ difference.
Is 38B bigger than 40A?
Same volume; 40A has the longer band and will feel looser.
Can I wear 40A instead of 38B?
Yes: that is what sister sizes are for. Move when the cups fit but the band is wrong.
What are 38B and 40A in UK and EU sizing?
38B: UK 38B, EU 85B, FR 100B. 40A: UK 40A, EU 90A, FR 105A.
Do 38B and 40A 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, 38B or 40A?
38B is one of the most widely stocked sizes; 40A is less common in mainstream stores.
Should I own both 38B and 40A?
Some people do: the snugger sister for structured, wired styles and the looser for soft or wireless ones. Same volume, so both fit the cups.
How do I measure to tell 38B from 40A?
Underbust snug (36.5–38.4″ gives band 38; 38.5–40.4″ gives 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.
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