Duga Fashion

38D vs 40C: The Difference, Measurements & Which Fits You

Bra Size Comparisons

Last updated: August 2026

Comparing 38D and 40C: these two are sister sizes — the same cup volume carried on a different band, so the choice is about how the band feels, not how much the cups hold.

Quick answer: 38D and 40C hold the same amount in the cup. Neither is ‘bigger’ — pick the band that sits level and snug.
How we work these sizes out. Every size on this page is calculated the same way: your band is your underbust measurement (taken snugly under the bust and rounded to the nearest even size), and your cup is the difference between your bust and underbust — about one cup per inch. It is the measurement-first method recommended by professional fitters and bra-fitting communities, and it is the same maths behind our bra size calculator.

38D vs 40C at a glance

  38D 40C
Band (underbust) 37–38 in (94–97 cm) 39–40 in (99–102 cm)
Bust 42–43 in (107–109 cm) 43–44 in (109–112 cm)
Cup (US) D C
UK 38D 40C
EU 85D 90C

The band difference

40C has the wider band and 38D the narrower one — a 2-inch difference around the ribcage. The band is where most of a bra’s support comes from, so this gap changes how the bra feels far more than the letter suggests.

The cup difference

On its own band, 38D takes the larger cup letter. Remember the letter is a bust-minus-band difference, though, so cup volume also depends on the band each letter sits on.

So which one is actually bigger?

Neither, in the cup. 38D and 40C are sister sizes: the cups hold the same volume and only the band width changes. If one feels tight around your body, the other is the natural swap — same cups, roomier or firmer band. This is the most useful thing to know about the two sizes, and it is why fitters move between sister sizes so often.

When to choose 38D over 40C

  • Pick 38D if the band on your current bra rides up at the back or feels loose — a firmer band sits better and supports more.
  • Pick 40C if your band digs in, leaves marks, or you struggle to breathe comfortably — the wider band relieves the pressure while keeping the same cup.

How to check the fit is actually right

Numbers get you to the right size on paper; these hands-on checks tell you whether it fits in real life. Do them in front of a mirror, wearing the bra on the loosest hook:

  • Band: it should sit level all the way around, in line with the front. Slide two fingers under it at the back — snug, not loose, no riding up when you raise your arms. The band does roughly 80% of the support work, so this matters most.
  • Cups: do a quick “scoop and swoop” — lean forward, settle everything into the cup, then stand up. There should be no spillage over the top or sides, and no wrinkling or gaping at the edge. Spillage means size up a cup; gaping means size down a cup.
  • Centre gore: the little bridge between the cups should lie flat against your breastbone. If it floats off the body, the cup is usually too small.
  • Straps: you should fit a finger under them comfortably. If they dig in or carry most of the weight, the band is too loose — tighten it or size the band down.

38D sister sizes

Size Underbust Bust
36DD 35–36 in 41–42 in
38D 37–38 in 42–43 in
40C 39–40 in 43–44 in

40C sister sizes

Size Underbust Bust
38D 37–38 in 42–43 in
40C 39–40 in 43–44 in
42B 41–42 in 44–45 in

International conversions

38D converts to 38D (UK) and 85D (EU); 40C converts to 40C (UK) and 90C (EU). Cup letters above D differ between US, UK and EU systems, so always check a brand’s own country of sizing.

These figures are a starting point, not a guarantee of fit. Cup lettering and cut vary between brands, and bodies differ in shape as well as measurement, so always try a size on where you can. This guide is for general information and is not medical advice — if you need fitting for a medical reason (for example after surgery), see a professional fitter or clinician.
Find your own size in seconds with the bra size calculator, or read the full 38D guide and full 40C guide.

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