40F Bra Size
40F fits a 39–41 inch (99–104 cm) underbust and a full bust of about 47 inches (119 cm). Its sister sizes are 38G and 42DD.
Last reviewed on 25 September 2026.
40F at a glance
| Underbust (band) | 39–41 in · 99–104 cm |
|---|---|
| Full bust | ≈ 47 in · 119 cm |
| Bust minus underbust | ≈ 7 in · 18 cm |
| Sister sizes (same cup volume) | 38G · 42DD |
| UK / EU / FR / AU | 40E · 90F · 105F · 18E |
| Same size in US | 40DDD |
| One cup bigger | 40G |
| One cup smaller | 40DD |
| Approx. top size | XL · UK 16–18 / US 12–14 |
Measurements for 40F
40F is built for an underbust ribcage measurement that falls in the 39–41 inch range, with a full-bust measurement around 47 inches. The band number — 40 — is a larger band where reinforced construction starts to matter for long-day comfort. The cup letter — F — describes the difference between the two measurements; a US F cup means the full bust is about seven inches larger than the underbust — it is the same cup as a US DDD (labels often read DDD/F) and a UK E. This is where mainstream lingerie sizing tends to thin out and specialist fuller-bust ranges take over.
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40F in Other Sizing Systems
EU, FR, and JP sizing read the band in centimetres, which is why 40F maps to 90F (EU), 105F (FR), and 90F (JP) — the same body, expressed in different units. AU sizing uses a small numeric scale (18E for this band). UK sizing shares the band number with the US but parts ways at the cup once you go past D, so 40F reads as 40E on a UK label.
Is 40F the same as 40DDD? In US sizing, yes: US brands use DDD and F for the same cup (labels often read "DDD/F"), so 40F and 40DDD are the same size. In UK sizing this cup is called 40E. If your 40F came from a UK brand, it is one cup larger — a UK 40F matches a US 40G. 40F is not the same as 40DD, which is one cup smaller.
Sister Sizes for 40F
These sizes have the same cup volume as 40F. 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 40F 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 40F
A US F cup is the same volume as a US DDD (UK E). Three-part cups, side support panels, and full-coverage shapes typically fit more cleanly than thin moulded cups at this size.
If your 40F 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 40F 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 40F is right for you.
40F — frequently asked questions
What measurements are a 40F bra in inches and cm?
A 40F bra fits an underbust (ribcage) measurement of roughly 39 to 41 inches (99–104 cm) and a full-bust measurement of about 47 inches (119 cm). The cup letter F reflects the roughly 7-inch (18 cm) difference between those two numbers.
What size is equivalent to 40F? (sister sizes)
The sister sizes of 40F are 38G (tighter band, one cup letter up) and 42DD (looser band, one cup letter down). They hold the same cup volume as 40F on a different band. Try a sister size when the cups of your 40F fit but the band feels too loose or too tight.
What is 40F in UK, EU, and other sizing systems?
40F (US) converts to 40E in UK sizing, 90F in EU sizing, 105F in French sizing, 18E in Australian sizing, and 90F in Japanese sizing. The body is the same; only the labelling system changes.
Is 40F a small or large bra size?
In terms of how common it is, this size pairs an above-average band with a fuller-bust 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 fuller end.
Is 40F 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 40F lands in the same letter size as other 40-band bras. With a fuller cup, many people go one size up in fitted tops so the bust has room. Letter sizing varies a lot between brands, so always check the brand's own chart.
What is the next size up or down from 40F?
Keeping the same 40 band, one size up in the cup is 40G, and one size down is 40DD. If you want a bigger or smaller band but the same cup volume, use a sister size instead: 38G or 42DD. Remember that the cup letter is relative to the band — 42F has more cup volume than 40F, not the same.
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