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40B Bras

(89 products)

How a 40B should fit

The band carries nearly all the support, so it should feel firm on the loosest hook and sit level all the way round. At B cup the more common fitting error is a band that's too loose paired with a cup that's too big — if the centre gore lifts away from your sternum, the cup is oversized, not the band.

Sister sizes to try

  • Band loose, cup fine — try 38C
  • Cup overflowing — try 40C
  • Band digs in — try 42A
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The HauteFlair Edit · Updated 2026

40B Bras: A Standard B Cup — the Same in US, UK and EU

A 40B is a straightforward size to shop across borders. The band is a 40 in every system, and a B cup is written B in US, UK and EU sizing alike — the systems don't diverge until well above the D range.

Below: exactly what 40B converts to across US, UK and EU, where the systems agree, how to measure to be sure, your sister sizes, and the styles that suit a B cup.

Quick Answer

What is a 40B bra size?

A 40B bra has a 40-inch underband and a B cup — and a B is written the same across systems. A UK 40B, US 40B and EU 90B all describe the same size: a bust roughly 2 inches larger than the band (around 42 inches around the fullest point). A, B and C sit well inside the range where US and UK sizing agree — they don't split until DDD/E — so it's an easy size to buy internationally. Sister sizes, holding the same cup volume, are 38C and 42A. Shop 40B bras in t-shirt, plunge, balconette, bralette, strapless and sports styles.

B = B = BSame letter in US, UK & EU
Band 40, cup B40 inches around the ribcage
≈ 2 inchesBust minus band at this cup
US & UK agreeThey don't split until DDD
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All 40B Bras T-Shirt Bras Strapless Sports Bras All Bras Size Conversion

How to choose a bra at 40B

A B cup is well-served by most standard bras — but these five checks still decide whether it fits cleanly or shifts through the day.

What to check What good looks like Why it matters
Band fit Level all the way around, snug on the loosest hook, two fingers under it and no more The band carries most of the support at any cup — a band that rides up shifts the load onto your shoulders
Cup shape Moulded for a smooth line, or a bralette for softness — both work well at a B A B is served by almost every construction; pick by the shape you want under clothes, not by necessity
Underwire Wires that sit flat on the ribcage and encircle the breast without poking the underarm — or skip the wire entirely A B is light enough that wire-free styles hold their shape, so comfort is a free choice here
Straps Adjustable, comfortable, staying put without being cranked tight At a B, standard straps are plenty; if they dig, the band is likely too loose
Gore & wings Centre gore flat to the breastbone; wings smooth against the side A floating gore means the cup is too small; smooth wings stop any side spillage

40B in US, UK, EU and beyond

The same body, seven labels — and at a B cup the letter doesn't move. The band number changes system to system; the cup stays B.

System Size How it's written
UK 40B The size on this page — the same letter everywhere
US 40B US and UK both write a 2-inch cup as B
EU 90B Band in centimetres; US 40 = EU 90. Cup still B
France / Spain 105B EU band plus 15 — same B cup, bigger number
Italy 5B Italy numbers bands; EU 90 = Italian 5
Australia / NZ 18B Numeric band (US 40 = AU 18), same B cup
Japan 90B EU band; Japanese cups run small, so size up one cup

So is a US B the same as a UK B? Yes — at a B cup they're identical, both meaning a bust about 2 inches larger than the band. The systems only split much higher up: a UK DD is still a US DD, but a UK E is a US DDD, and a UK F is a US G. A B is nowhere near that split. See our full bra size conversion guide.

Find your 40B bra

Pick the card that matches what you need it for — then tap through to shop.

01

Everyday, under everything

A moulded t-shirt bra: smooth under knit, rounded shape, no seam showing through.

Shop t-shirt bras →
02

Shape and lift

A plunge or balconette adds shape and opens the neckline — the going-out pick at this cup.

Shop all bras →
03

Running, lifting, moving

A B is well covered by compression or lightly encapsulated sports bras. Pick by impact level.

Shop sports bras →
04

Strapless that stays

A B is very wearable strapless — look for a firm band and silicone grip and it holds.

Shop strapless →
05

Soft and wire-free

At a B, a bralette holds its shape — the easy comfortable option for low-key days.

Shop bralettes →
06

Not sure it's your size?

If your current 40B gapes or the gore floats, the conversion or your measurements may be off. Start here.

Convert my size →

The band does the lifting; the straps only steady it. A B cup is easy to fit well once the band is right — the gaping, the ride-up and the digging usually trace back to the band, not the cup.

The HauteFlair Standard

The complete 40B sizing guide

Measuring, where the systems agree, sister sizes, the fit tells and the vocabulary.

01How to measure for a 40B at home+

Two measurements, taken in a soft unpadded bra or none at all, tape level:

  • Band: snug around the ribcage directly under the bust. A 40 band comes from an underbust near 39–41 inches.
  • Bust: around the fullest point, tape parallel to the floor, not pulled tight. At 40B that lands around 42 inches.
  • Cup: subtract band from bust. A 2-inch difference is a B; A and C are a cup either side.

The result is a starting size, not a verdict. Cut varies mill to mill, so let the fit checks below decide — and if you're on the line between letters, try the size plus its sister sizes.

02Where US and UK sizing agree — and where they split+

A B is safely inside the zone where both systems match. Here's the ladder:

  • They agree through DD: A, B, C, D, DD read the same in US and UK. A 40B is a 40B in both.
  • They split at DDD: above DD the US stacks another D (DDD) while the UK moves to E — so a UK E is a US DDD.
  • From there they diverge: UK F is a US G, UK FF is a US H, and so on up the alphabet.

So a B cup is one you never have to convert. It's only above the D range that you need to check whether a chart is UK or US before trusting the letter.

03Sister sizes: 38C, 40B, 42A+

Cup volume moves with the band, so 40B has neighbours holding roughly the same cup:

  • Down a band, up a cup: 38C — for a firmer band that feels tighter.
  • Up a band, down a cup: 42A — for a looser band with the same cup room.

Reach for a sister size when the cup fits but the band doesn't, or when your exact size is out of stock in a style you want. What you should never do is loosen the band to fix the cup — that hands the support to your shoulders. The full logic is in our sister sizes guide.

04The five fit tells at a B cup+

Stand at a mirror. In order of usefulness:

  • Band riding up at the back — band too loose. Go down a band, up a cup (your sister size).
  • Gore floating off the breastbone — cup too small, or the wire is too narrow for you.
  • Spillage at the top, underarm or under the wire — cup too small. Not a shape problem.
  • Wrinkling or gaping at the top of the cup — cup too big, or the wrong style for your shape.
  • Straps digging — you've tightened them to cover for a loose band. Fix the band first.

Check on the loosest hook when the bra is new; you move to the tighter hooks as the elastic relaxes over time.

0540B, B cup, 90B: the vocabulary+
  • 40B — a 40 band with a B cup; the same in US and UK sizing.
  • EU 90B — the same body; 90 is the band in centimetres, and the cup is still B.
  • Standard cup — A, B and C are the everyday cups, stocked across nearly every style and brand.
  • Sister size — a different band-and-cup pairing holding the same cup volume; 40B's are 38C and 42A.
  • The split — above DD, US and UK letters diverge (UK E = US DDD). A B is well below it.

40B bras FAQ

The questions shoppers ask most, answered straight.

What is a 40B bra size?

A 40B has a 40-inch underband and a B cup. In US, UK and EU sizing a B cup is written the same way — it's a bust about 2 inches larger than the band (around 42 inches around the fullest point), which makes 40B a US 40B, a UK 40B and an EU 90B. It's a standard, widely stocked cup, well inside the range where the sizing systems agree.

Is 40B a big or small bra size?

A B is one of the standard everyday cups, on the smaller-to-middle side of the range, but the letter alone doesn't settle it. Cup volume is measured against the band, so a 40B holds more than a 38B (smaller band) and less than a 42B (larger band) even though all three read B. The band is half the size.

Is 40B a UK or US size?

Either — at a B cup it's the same. US and UK sizing agree all the way up to DD, and a B is well below that, so a UK 40B and a US 40B are identical. The two systems only split much higher up: a UK E is a US DDD, and a UK F is a US G.

What is a 40B in US sizing?

Also 40B. A 2-inch difference between bust and band is a B cup in both US and UK sizing, so the letter doesn't change. Only the larger UK cups (FF, GG) get rewritten in US letters — a B stays a B.

What is a 40B in EU sizing?

A 40B is an EU 90B. The band converts from inches to centimetres (40 inches = EU 90), but the cup letter stays B — A, B and C land on the same letter in US, UK and EU sizing. In France or Spain the same size reads 105B.

Is a 40B bigger than a 40A?

Yes, by one cup. On the same 40 band, each step up the alphabet adds cup volume, so a 40B is one cup larger than a 40A and one cup smaller than a 40C. If a A gapes at the top or a C spills over, B is the size in between.

What's the difference between 40B and 40C?

One cup. A C holds about an inch more projection than a B on the same band. If your 40B cup cuts in, spills over the top or the underarm, the next size up is 40C — not a bigger band.

What are the sister sizes for 40B?

38C and 42A — the same cup volume on a different band. Drop to 38C for a firmer band, move up to 42A for a looser one. Use a sister size when the cup fits but the band doesn't, or when your size is sold out in the style you want.

How do I measure for a 40B at home?

Measure snugly around your ribcage under the bust for the band, then around the fullest point with the tape level. Subtract the first from the second: a 2-inch difference on a 40 band points to a B cup. Treat it as a starting size and let the fit checks decide.

What bra styles work best for a 40B?

Most styles — a B is a well-served cup, so t-shirt, plunge, balconette, bralette, strapless and sports styles all come in it. A moulded t-shirt bra works smoothly under knits, a plunge or balconette shapes and lifts, and a wire-free bralette is an easy comfortable option at this cup.

Sizing guidance here is general. Bra sizing is not standardised — cup labelling, wire shape and band firmness vary by brand and style, so conversions are a starting point, not a guarantee. Always check the individual brand's size chart and treat any measurement as a place to begin. Size availability reflects current stock. Last reviewed August 2026.

Shop 40B bras

A B cup that reads the same in US, UK and EU — in t-shirt, plunge, balconette, bralette, strapless and sports styles, with bands that fit and cups that hold their shape.

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