AU to US conversion

AU 18A in US Bra Size

Convert 18A from Australian sizing to US sizing, with the full international table, sister sizes, and practical fit checks in one place.

Instant answer

18A (AU) =

40A US

In this sizing table, 18A in Australian sizing converts to 40A in US 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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Full conversion table for 18A (US 40A)

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

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

What this conversion means

Reverse conversions are useful when a tag, retailer, or size chart starts outside the US system. In this table, AU 18A converts to US 40A, then the adjacent links help you compare nearby options.

Sister sizes for the US equivalent (40A)

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

What measurements make a 18A?

18A 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 40.5-41.5 inches (103-105 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 40A as the US equivalent for those measurement notes.

Is 18A big or small?

The label 18A 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 18A 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 18A differs between brands

Brand sizing matters for 18A 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 18A

A good shopping path for 18A 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.

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Everyday Fit Store

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Comfort Band Co.

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Soft Cup Edit

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FAQ

Is 18A the same as 40A in the US?

In this dataset, 18A (AU) converts to 40A 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 18A in European sizing?

18A (AU) matches the same base record as 90A (EU). The full table on this page shows each supported label side by side.

How do I check the fit of 40A?

Start with the band: it should feel firm and even around your ribcage. Then check cup containment, wire comfort, and whether the center gore lies close on wired styles.

What sister sizes work with 40A?

38B are the closest sister-size options for 40A 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 18A?

18A maps to US 40A here. The current calculator formula points to about 39-41 inches underbust and 40.5-41.5 inches bust, or roughly 99-104 cm and 103-105 cm. Treat that as a starting range and then check the real fit.