40B Bra Size
Everything about the 40B bra size in United States sizing: what it means, its measurements, whether it runs big or small, sister sizes, and conversions to UK, EU, FR & AU.
What is a 40B bra size?
A 40B in United States sizing is a bra with a 40 band and a B cup. “40” is the band in inches, taken from a snug underbust measurement rounded to the nearest even number. “B” is the cup, from a 2-inch difference between the bust and the band (one inch per cup letter). The same body is written as UK 40B, EU 90B, FR 105B, AU 18B, IT 6B.
Someone who measures roughly 39 inches (99 cm) around the ribcage and about 42 inches (107 cm) around the fullest part of the bust lands at 40B using the modern snug-measure method. The band carries most of the support; the cup describes the breast volume the bra needs to hold: measured relative to that band.
“40” is the band in inches, taken from a snug underbust measurement rounded to the nearest even number. It sets how the bra fits around the ribcage and provides about 80% of the support.
B is a modest B-cup: a 2-inch bust-minus-band difference. It reflects breast volume, always relative to the 40 band.
Band and cup are one size. A 40B is not the same volume as a 36B: the cup letter only has meaning next to its band.
40B measurements
A 40B comes from two measurements: a snug underbust that gives the 40 band, and a looser bust. The cup is the gap between the two: for B, that gap is 2 inches.
| Measurement | Range | Sets |
|---|---|---|
| Underbust (snug) | 38.5–40.4″ · 98–103 cm | Band 40 |
| Bust (fullest) | 41.5–42.4″ · 105–108 cm | — |
| Difference | 2″ · ≈ 5 cm | Cup B |
Underbust rounds to the nearest even band; an odd reading (e.g. 39″) can go either way, so try 38C and 40B and let the band decide.
Measurements vary by body shape, so treat these as the center of a range.
Is 40B big or small?
40B is a smaller-volume cup on a fuller band: a commonly stocked size. But the honest answer is that a cup letter is not a fixed amount of volume: it grows with the band. Tap a size below to see how.
All five are B cups, yet the volume grows with the band. 40B is the highlighted size.
Because the cup is measured relative to the band, a 40B holds more than a 36B and less than a 44B, even though all three are “B cups.” So “big” or “small” only has meaning once you know the band.
If 40B cups fit well but the band feels off, you don't need a different cup: you need a sister size. And if you're unsure 40B is your starting size, the calculator settles it from two measurements.
Were you measured as 40B correctly?
Most people wearing the wrong size were sized by the old “+4 rule”: adding four inches to the underbust to get the band. It inflates the band and, because the cup is the difference from that inflated band, understates the cup by about two letters.
Same body, same volume: the old rule pushes it into a looser band and a smaller cup.
The tell-tale signs you were sized with the old method: the 40 band rides up at the back, you fasten it on the tightest hook from day one, and the straps do most of the work. If that sounds familiar, 36D is the size to try first: it holds the same volume on a band that actually supports.
Key takeaway: if a calculator adds inches to your underbust, its 40B is not the same 40B as ours. Brasize.ai uses the modern snug-measure standard used by professional fitters.
Which bra styles fit a 40B best?
Shallow-to-modest cups suit plunge, demi and balconette shapes that create lift and shape without excess fabric; wireless and bralette styles also work because there's little weight to support. Avoid full-coverage cups cut for projection: they tend to gape at the top on a shallow profile. On a 40 band, prioritise firm, wide band elastic and a taller back; a soft, narrow band will roll and ride up regardless of the label. Longline and three-hook styles help.
Shallow, lifting shapes: plunge, demi, balconette.
Look for a three-hook band with firm, wide elastic: the band does the lifting.
Standard wires; make sure the wire sits on the ribcage at the side, not on tissue.
Sister sizes share cup volume but not cup shape: the wire on a smaller band is narrower and more projected, the wire on a larger band is wider and shallower. If you have a narrow breast root, the snugger sister will often fit better; if your root is wide, the looser sister may: even when the band tension of the original was right.
40B sister sizes
Sister sizes hold the same cup volume as 40B on a different band. They're the fix when the cups fit but the band is too loose or too tight.
| Size | Band feel vs 40B | Cup volume | When to try it |
|---|---|---|---|
| 36D | Two bands snugger | Same as 40B | Only if 38C is still loose; wire will be noticeably narrower |
| 38C | One band snugger | Same as 40B | 40 band rides up or feels loose |
| 40B | Your starting size | — | Band sits firm and level |
| 42A | One band looser | Same as 40B | 40 band digs in or feels tight |
The rule: go down a band and up a cup (40B → 38C) for the same cups on a tighter band, or up a band and down a cup (40B → 42A) for the same cups on a looser band. Sister sizes share cup volume but not cup shape: the wire on a smaller band is narrower and more projected, the wire on a larger band is wider and shallower. If you have a narrow breast root, the snugger sister will often fit better; if your root is wide, the looser sister may: even when the band tension of the original was right.
Signs 40B is too small, or too big
Fix: go up a cup on the same band → 40C. If the band is also loose, go to 38C.
Fix: gaping cups → down a cup, 40A. Band riding up with cups that fit → 38C.
40B in UK, EU, FR & AU sizing
The same 40B body measurements are written differently around the world. Bands and cup letters diverge by region, so convert deliberately when buying from an international brand.
| Region | Size | How the band is derived |
|---|---|---|
| United States (US) | 40B | Band in inches |
| United Kingdom (UK) | 40B | Same band as US; cups diverge above DD |
| Europe (EU) | 90B | US band × 2.5 − 10 = 90 |
| France / Spain (FR) | 105B | EU band + 15 = 105 |
| Australia / NZ (AU) | 18B | US band − 22 = 18 |
| Italy (IT) | 6B | (EU − 60) ÷ 5 = 6 |
| Japan (JP) | 90B | EU band; cups close to EU |
| Germany (DE) | 90B | EU sizing |
Cup letters agree across systems up to D. Above D they diverge (a US DDD is a UK E and an EU F) which is where the size converter earns its keep. Brands that size natively in US: Natori, Wacoal, Le Mystère, Victoria’s Secret.
How to tell if 40B is your correct size
A calculated 40B is an accurate starting point. Confirm it on the body: band, cups, gore, straps.
- Band sits firm and level. The 40 band should be snug around your ribcage with room for two fingers, not riding up at the back. It gives most of the support.
- Cups fully contain the breast. No spillage over the top or sides, and no gaping or wrinkling in the B cups. Gaping usually means down a cup; spillage means up a cup.
- Center gore lies flat. The panel between the cups should sit against your sternum, not float away.
- Straps stay put. If you're tightening straps to feel supported, the 40 band is probably too big: try 38C.
If the cups fit but the band is wrong, move to a sister size rather than changing cup letters. There's no universal standard between brands: a 40B in one label can fit like the size on either side in another, so use the fit checks, not just the label. Full symptom-by-symptom guide: bra fit guide.
Compare this size
40B against the sizes people confuse it with: one cup apart on the same band, and its sister sizes.
Educational resource, not medical advice. Read our editorial policy.
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.
Is 40B actually your size?
Don't guess from a label. Enter two measurements and get your accurate size in eight systems, plus your sister sizes.
Calculate My Bra Size →40B bra size: answered
The questions people ask most about the 40B size: its volume, sister sizes, conversions and fit.
Is 40B a big bra size?
40B is a modest B-cup on a 40 band. A cup letter is a ratio, not a fixed volume: B means a 2-inch difference between bust and band, so a 40B holds more than a 36B and less than a 44B. “Big” only has meaning once you know the band.
What is the sister size of 40B?
The sister sizes of 40B are 38C (snugger band) and 42A (looser band). Sister sizes hold the same cup volume as 40B on a different band. Move to one when the B cups fit but the 40 band is too loose or too tight.
What is 40B in European sizing?
A US 40B is an EU 90B. EU bands follow US × 2.5 − 10; cup letters match up to D, then EU continues the alphabet (E, F, G) where the US repeats D.
What is 40B in UK sizing?
A US 40B is a UK 40B. US and UK share inch bands and agree on cups through DD; above that US DDD = UK E, US G = UK F, US H = UK FF.
What measurements are a 40B?
Roughly a 39-inch (99 cm) snug underbust and a 42-inch (107 cm) bust at the fullest point. The 2-inch difference gives the B cup; the underbust gives the 40 band. Treat these as the center of a range.
What is 40B in centimeters?
A 40B corresponds to an underbust of about 98–103 cm and a bust of about 105–108 cm: a difference of roughly 5 cm. In metric-labelled systems the same size is EU 90B, FR 105B and JP 90B.
Which bra styles suit a 40B?
Shallow-to-modest cups suit plunge, demi and balconette shapes that create lift and shape without excess fabric; wireless and bralette styles also work because there's little weight to support. Avoid full-coverage cups cut for projection: they tend to gape at the top on a shallow profile. On a 40 band, prioritise firm, wide band elastic and a taller back; a soft, narrow band will roll and ride up regardless of the label. Longline and three-hook styles help.
How do I know if 40B is my correct size?
Check four things: the 40 band sits level and snug on the loosest hook, the B cups are filled without spillage or gaping, the center gore lies flat against the sternum, and the straps stay put without carrying the weight. If the band is wrong but the cups fit, try a sister size such as 38C.
Was I sized as 40B using the old +4 method?
If your 40 band rides up, fastens on the tightest hook from new, and the straps carry the weight, probably yes. The +4 rule adds four inches to the underbust; the same body measured snugly is closer to 36D, same cup volume, a band that supports.