FR to US conversion

FR 100B in US Bra Size

Convert 100B from French sizing to US sizing, with the full international table, sister sizes, and practical fit checks in one place.

Instant answer

100B (FR) =

38B US

In this sizing table, 100B in French sizing converts to 38B in US sizing.

Full conversion table for 100B (US 38B)

38B bra size conversion across US, UK, EU, French, and Australian sizing.
US	UK	EU	FR	AU
38B	38B	85B	100B	16B

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

What this conversion means

The US equivalent for 100B is 38B here. If the converted cup volume feels close but the band feels too snug or loose, use the sister sizes below before jumping several cup letters.

Sister sizes for the US equivalent (38B)

Sister sizes: 36C and 40A can be worth checking when the cup volume feels close but the band needs a different feel.

What measurements make a 100B?

A size like 100B makes more sense when the two tape measurements are shown side by side. In the current calculator formula, the closest starting range is about 37-39 inches (94-99 cm) under the bust and about 39.5-40.5 inches (100-103 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 38B as the US equivalent for those measurement notes.

Is 100B big or small?

100B should be treated as a fit label, not as a judgement about body size. 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 100B 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 100B differs between brands

Use 100B as the starting label, then let the individual brand chart narrow the choice. 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 100B

Treat 100B as your anchor size while you compare cup shape, strap placement, and band feel. 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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Comfort Band Co.

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Soft Cup Edit

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FAQ

Is 100B the same as 38B in the US?

In this dataset, 100B (FR) converts to 38B 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 100B in UK sizing?

100B (FR) matches the same base record as 38B (UK). The full table on this page shows each supported label side by side.

How do I check the fit of 38B?

A good US fit should feel secure without digging. If 38B feels close in the cup but wrong in the band, compare the sister sizes before changing cup volume.

What sister sizes work with 38B?

36C and 40A are the closest sister-size options for 38B 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 100B?

100B maps to US 38B here. The current calculator formula points to about 37-39 inches underbust and 39.5-40.5 inches bust, or roughly 94-99 cm and 100-103 cm. Treat that as a starting range and then check the real fit.