40C Bra Size
Everything you need to know about bra size 40C — measurements, sister sizes, international equivalents, and fit advice.
Last reviewed on 28 April 2026.
Measurements for 40C
40C is built for an underbust ribcage measurement that falls in the 39–41 inch range, with a full-bust measurement around 44 inches. The band number — 40 — is a larger band where reinforced construction starts to matter for long-day comfort. The cup letter — C — describes the difference between the two measurements; a C cup means the full bust is about three inches larger than the underbust — this is the threshold where cup shape (full-cup, demi, balconette, plunge) starts to make a noticeable difference.
40C in Other Sizing Systems
EU, FR, and JP sizing read the band in centimetres, which is why 40C maps to 90C (EU), 105C (FR), and 90C (JP) — the same body, expressed in different units. AU sizing uses a small numeric scale (18C for this band). UK sizing shares the band number with the US but parts ways at the cup once you go past D, so 40C reads as 40C on a UK label.
Sister Sizes for 40C
These sizes have the same cup volume as 40C. 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 40C 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 40C
C cups work well across most bra styles. If you spill over in one brand but fit fine in another, cup shape and depth vary by maker — the size on the label is only part of the story.
If your 40C 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 40C 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 40C is right for you.
40C — frequently asked questions
What measurements are a 40C bra?
A 40C bra fits an underbust (ribcage) measurement of roughly 39 to 41 inches (99–104 cm) and a full-bust measurement of about 44 inches (112 cm). The cup letter C reflects the difference between those two numbers.
What are the sister sizes of 40C?
The sister sizes of 40C are 38D and 42B. They share the same cup volume as 40C but use a different band — go to 38D for a tighter band, or up for a looser one. Try a sister size when the cup of your 40C fits but the band feels too loose or too tight.
What is 40C in UK, EU, and other sizing systems?
40C (US) converts to 40C in UK sizing, 90C in EU sizing, 105C in French sizing, 18C in Australian sizing, and 90C in Japanese sizing. The body is the same; only the labelling system changes.
Is 40C 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.
Not sure about your size? Measure yourself and double-check.
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