US to AU conversion

40B in Australian Bra Size

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

Instant answer

40B (US) =

18B AU

In this sizing table, 40B in US sizing converts to 18B in Australian sizing.

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If you are starting from body measurements instead of a known label, use the AU/NZ calculator with centimetres selected by default.

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Not sure about your size? Recheck your measurements before comparing conversion labels.

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Sizing varies by brand

A conversion gives you the closest label in another sizing system, but brands can grade cups, wires, and bands differently. Use the converted size as a starting point, then check the retailer's chart and the fit of the actual bra.

Before you buy in Australian sizing

Use 18B as the translated label, not as a fit guarantee.
Check the retailer's size chart because some brands grade bands and cups differently.
If the cup volume feels close but the band changes, compare sister sizes before switching cup letters.
When possible, check return rules before buying across sizing systems.

Full conversion table for 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.

Sizes near 40B in Australian sizing

If 40B is close but not right, these are the neighbouring US sizes and what each one becomes in Australian sizing. Band neighbours change the fit around the ribcage; cup neighbours change volume.

US sizes adjacent to 40B and their Australian equivalents.
US size	AU size	How it differs
38B	16B	One band step snugger, same cup letter.
40A	18A	One cup step down, same band.
40C	18C	One cup step up, same band.

What this conversion means

For 40B, the band stays tied to the 40 US band while the label is translated into Australian sizing. In this dataset, 40B (US) converts to 18B (AU). Treat that as the starting label, then check the brand chart and the real fit.

Sister sizes for 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?

The useful way to read 40B is to separate the band from the cup step. 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.

Is 40B big or small?

Whether 40B feels small, average, or full depends on the body and on the brand. 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

40B can shift in feel from one retailer to another because brands grade bands and cups in their own way. 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

40B is easiest to compare when you try shapes that show both cup volume and band tension clearly. 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

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Soft Cup Edit

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FAQ

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

The closest Australian label here is 18B. It is a conversion guide, so retailer charts and try-on fit still matter.

What is 40B in French sizing?

In this same table, 40B converts to 105B (FR). That helps when a retailer shows a French chart instead of a Australian one.

How do I know if 40B fits?

Start with the band: it should feel firm and even around your ribcage. Then check cup containment and comfort across the style you are actually trying on.

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?

Countries and brands do not all use the same band labels or cup-letter sequence. A conversion translates the closest standard label, but the final fit still depends on the specific bra.

What measurements usually make 40B?

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.