40D Bra Size
40D fits a 39–41 inch (99–104 cm) underbust and a full bust of about 45 inches (114 cm). Its sister sizes are 38DD and 42C.
Last reviewed on 25 September 2026.
40D at a glance
| Underbust (band) | 39–41 in · 99–104 cm |
|---|---|
| Full bust | ≈ 45 in · 114 cm |
| Bust minus underbust | ≈ 5 in · 13 cm |
| Sister sizes (same cup volume) | 38DD · 42C |
| UK / EU / FR / AU | 40D · 90D · 105D · 18D |
| One cup bigger | 40DD |
| One cup smaller | 40C |
| Approx. top size | XL · UK 16–18 / US 12–14 |
Measurements for 40D
40D is built for an underbust ribcage measurement that falls in the 39–41 inch range, with a full-bust measurement around 45 inches. The band number — 40 — is a larger band where reinforced construction starts to matter for long-day comfort. The cup letter — D — describes the difference between the two measurements; a D cup means the full bust is about five inches larger than the underbust — at this volume, unlined cups often give a more natural shape than heavily padded ones.
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40D in Other Sizing Systems
EU, FR, and JP sizing read the band in centimetres, which is why 40D maps to 90D (EU), 105D (FR), and 90D (JP) — the same body, expressed in different units. AU sizing uses a small numeric scale (18D for this band). UK sizing shares the band number with the US but parts ways at the cup once you go past D, so 40D reads as 40D on a UK label.
Sister Sizes for 40D
These sizes have the same cup volume as 40D. Try them if the band feels too tight or too loose.
← Tighter band, bigger cup · Looser band, smaller cup →
Sister sizes are useful when the cup of your 40D fits well but the band is wrong — too loose by the end of the day, or too tight to clip up comfortably. Going one band smaller and one cup larger keeps the cup volume the same while tightening the band; going the other way loosens the band without losing cup space. Read the full sister-size matrix if you want to see how this works for every size at once.
Fit Tips for 40D
D cups benefit from wider straps for comfort. Unlined or lightly-lined cups give a more natural shape at this size; very heavily padded cups can over-shape.
If your 40D bra is gapping at the top of the cup, the cup may be too large or the wrong shape — try a demi-cup or balconette before assuming the size is wrong. If the centre piece between the cups (the gore) does not lay flat against your sternum, the cup is almost certainly too small; go up one cup. If the band rides up across your back during the day, the band has stretched out or was too loose to begin with — drop one band size and add one cup to keep the same cup volume.
For more detailed troubleshooting, the Fit Test on the home page covers six common visible problems and the size or shape change that usually addresses each one. The guide to measuring mistakes covers the most common reasons a calculation comes out wrong in the first place.
Related sizes
If 40D is close but not quite right, these neighbouring sizes are the most useful next places to look:
See every size at a glance on the full bra size index, or read how to measure your bra size if you are not sure 40D is right for you.
40D — frequently asked questions
What measurements are a 40D bra in inches and cm?
A 40D bra fits an underbust (ribcage) measurement of roughly 39 to 41 inches (99–104 cm) and a full-bust measurement of about 45 inches (114 cm). The cup letter D reflects the roughly 5-inch (13 cm) difference between those two numbers.
What size is equivalent to 40D? (sister sizes)
The sister sizes of 40D are 38DD (tighter band, one cup letter up) and 42C (looser band, one cup letter down). They hold the same cup volume as 40D on a different band. Try a sister size when the cups of your 40D fit but the band feels too loose or too tight.
What is 40D in UK, EU, and other sizing systems?
40D (US) converts to 40D in UK sizing, 90D in EU sizing, 105D in French sizing, 18D in Australian sizing, and 90D in Japanese sizing. The body is the same; only the labelling system changes.
Is 40D a small or large bra size?
In terms of how common it is, this size pairs an above-average band with a mid-range cup. There is no universal "small" or "large" bra size — the same cup letter describes a different volume on every band — but within the full range BraCalc covers (bands 28–48, cups AA–H) this combination sits toward the middle end.
Is 40D a small, medium, large, or XL?
As a rough guide, a 40 band usually matches size XL in tops and bralettes — around UK dress size 16–18 (US 12–14). The band number drives this more than the cup letter, so 40D lands in the same letter size as other 40-band bras. Letter sizing varies a lot between brands, so always check the brand's own chart.
What is the next size up or down from 40D?
Keeping the same 40 band, one size up in the cup is 40DD, and one size down is 40C. If you want a bigger or smaller band but the same cup volume, use a sister size instead: 38DD or 42C. Remember that the cup letter is relative to the band — 42D has more cup volume than 40D, not the same.
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