UK to US conversion

UK 40B in US Bra Size

Convert 40B from UK sizing to US sizing, with the full international table, sister sizes, and practical fit checks in one place.

Instant answer

40B (UK) =

40B US

In this sizing table, 40B in UK sizing converts to 40B in US sizing.

Full conversion table for 40B (US 40B)

40B bra size conversion across US, UK, EU, French, and Australian sizing.
US	UK	EU	FR	AU
40B	40B	90B	105B	18B

Bra sizing varies between brands. Use this as an accurate starting point and check the fit.

What this conversion means

UK sizing can look unfamiliar because the band and cup sequences are not always written like US labels. For 40B, this shared data file maps the fit target to 40B in US sizing.

Sister sizes for the US equivalent (40B)

Sister sizes: 38C can be worth checking when the cup volume feels close but the band needs a different feel.

What measurements make a 40B?

40B starts with the underbust, then adds cup depth from the bust measurement. In the current calculator formula, the closest starting range is about 39-41 inches (99-104 cm) under the bust and about 41.5-42.5 inches (105-108 cm) around the fullest part of the bust. The band comes from the underbust rounding, while the cup step comes from the difference between bust and band. This page uses 40B as the US equivalent for those measurement notes.

Is 40B big or small?

The label 40B can look more dramatic than it fits, especially across systems. In this size set, the US equivalent uses a fuller band and a lower cup step. Cup letters are relative to the band, so a D on one band is not the same volume as a D on every other band. The most useful question is whether the band stays level and the cups hold smoothly, not whether the label sounds large or small.

If 40B doesn't fit right

Band rides up: The band may be too loose or stretched out; try a snugger band or compare the snugger sister size.

Cups gape: The cup may be too roomy, too deep, or pulled away by a loose band; check the band before changing cup volume.

Cups spill: The cup volume may be too small or the cup shape may be too closed; try one cup step up before changing the band.

Straps dig in: The straps may be compensating for a band that is not supporting enough; reset the band fit before tightening straps.

How 40B differs between brands

Brand sizing matters for 40B because cup depth, wire width, and band stretch are design choices. A firmer elastic can make the same band feel smaller, and a shallow or narrow cup can make the same cup letter feel different. Check the retailer chart, read fit notes when available, and compare the label against the brand and region sizing guide before treating the conversion as final.

Shopping for 40B

A good shopping path for 40B starts with everyday support, then moves into outfit-specific shapes. If the cup is in a lower or middle step, lightly lined T-shirt styles can make gaps easy to spot. Fuller cup steps often benefit from styles with steadier side support, deeper cups, or a balconette shape depending on breast shape. For special outfits, compare the fit needs in the outfit guides before choosing a strapless, plunge, or backless option.

Bras in your size

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Everyday Fit Store

Placeholder for a broad size range and return-friendly basics.

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Comfort Band Co.

Placeholder for supportive everyday styles by band and cup.

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Soft Cup Edit

Placeholder for wire-free and lounge options.

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FAQ

Is 40B the same as 40B in the US?

In this dataset, 40B (UK) converts to 40B in US sizing. Some brands publish their own charts, so use this as the closest standard label and still check the real fit.

What is 40B in Australian sizing?

40B (UK) matches the same base record as 18B (AU). The full table on this page shows each supported label side by side.

How do I check the fit of 40B?

A good US fit should feel secure without digging. If 40B feels close in the cup but wrong in the band, compare the sister sizes before changing cup volume.

What sister sizes work with 40B?

38C are the closest sister-size options for 40B in this dataset. Try a sister size when cup volume feels close but the band needs to feel snugger or looser.

Why do bra sizes differ between countries?

Bra labels are written differently across US, UK, EU, French, and Australian systems. Cup letters can also diverge after D, so a converted label is a starting point rather than a promise of identical fit.

What measurements usually make 40B?

40B maps to US 40B here. The current calculator formula points to about 39-41 inches underbust and 41.5-42.5 inches bust, or roughly 99-104 cm and 105-108 cm. Treat that as a starting range and then check the real fit.