AU to US conversion

AU 18DD in US Bra Size

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

Instant answer

18DD (AU) =

40DD US

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

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

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

What this conversion means

Australian sizing can look unfamiliar because the band and cup sequences are not always written like US labels. For 18DD, this shared data file maps the fit target to 40DD in US sizing.

Sister sizes for the US equivalent (40DD)

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

What measurements make a 18DD?

18DD is built from a band measurement and a bust measurement, not from the label alone. In the current calculator formula, the closest starting range is about 39-41 inches (99-104 cm) under the bust and about 44.5-45.5 inches (113-116 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 40DD as the US equivalent for those measurement notes.

Is 18DD big or small?

18DD is not automatically big or small; it depends on the band and the cup together. In this size set, the US equivalent uses a fuller band and a fuller 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 18DD 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 18DD differs between brands

Two brands can print 18DD or its equivalent on the tag and still cut the bra differently. 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 18DD

When shopping for 18DD, start with styles that make the band easy to judge. 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 18DD the same as 40DD in the US?

In this dataset, 18DD (AU) converts to 40DD 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 18DD in French sizing?

18DD (AU) matches the same base record as 105E (FR). The full table on this page shows each supported label side by side.

How do I check the fit of 40DD?

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

What sister sizes work with 40DD?

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

18DD maps to US 40DD here. The current calculator formula points to about 39-41 inches underbust and 44.5-45.5 inches bust, or roughly 99-104 cm and 113-116 cm. Treat that as a starting range and then check the real fit.