AU to US conversion

AU 18D in US Bra Size

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

Instant answer

18D (AU) =

40D US

In this sizing table, 18D in Australian sizing converts to 40D 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 18D (US 40D)

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

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

What this conversion means

This reverse conversion starts with the label you already have: 18D (AU). The matching US record is 40D, and the table shows the same size across UK, EU, French, and Australian systems for easier retailer comparison.

Sister sizes for the US equivalent (40D)

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

What measurements make a 18D?

For 18D, the band number gives the ribcage anchor and the cup describes the bust difference. In the current calculator formula, the closest starting range is about 39-41 inches (99-104 cm) under the bust and about 43.5-44.5 inches (110-113 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 40D as the US equivalent for those measurement notes.

Is 18D big or small?

18D sits in context only when the number and letter are read as a pair. In this size set, the US equivalent uses a fuller band and a middle 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 18D 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 18D differs between brands

A conversion points you to a label, but a brand chart tells you how that maker expects 18D to fit. 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 18D

For 18D, practical shopping is less about one perfect style and more about reading the fit quickly. 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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FAQ

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

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

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

How do I check the fit of 40D?

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 40D?

38DD are the closest sister-size options for 40D 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 18D?

18D maps to US 40D here. The current calculator formula points to about 39-41 inches underbust and 43.5-44.5 inches bust, or roughly 99-104 cm and 110-113 cm. Treat that as a starting range and then check the real fit.