40AA Bra Size
40AA fits a 39–41 inch (99–104 cm) underbust and a full bust of about 41 inches (104 cm). Its sister sizes are 38A.
Last reviewed on 25 September 2026.
40AA at a glance
| Underbust (band) | 39–41 in · 99–104 cm |
|---|---|
| Full bust | ≈ 41 in · 104 cm |
| Bust minus underbust | ≈ 1 in · 3 cm |
| Sister sizes (same cup volume) | 38A |
| UK / EU / FR / AU | 40AA · 90AA · 105AA · 18AA |
| One cup bigger | 40A |
| Approx. top size | XL · UK 16–18 / US 12–14 |
Measurements for 40AA
40AA is built for an underbust ribcage measurement that falls in the 39–41 inch range, with a full-bust measurement around 41 inches. The band number — 40 — is a larger band where reinforced construction starts to matter for long-day comfort. The cup letter — AA — describes the difference between the two measurements; an AA cup means the full bust is only about one inch larger than the underbust — there is little projection, and the cup mostly serves as smooth coverage rather than tissue support.
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40AA in Other Sizing Systems
EU, FR, and JP sizing read the band in centimetres, which is why 40AA maps to 90AA (EU), 105AA (FR), and 90AA (JP) — the same body, expressed in different units. AU sizing uses a small numeric scale (18AA for this band). UK sizing shares the band number with the US but parts ways at the cup once you go past D, so 40AA reads as 40AA on a UK label.
Sister Sizes for 40AA
These sizes have the same cup volume as 40AA. 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 40AA 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 40AA
AA cups can be hard to find in mainstream stores. Brands and ranges focused on petite or smaller-cup fitting tend to carry more options, often in soft-cup, bralette, or triangle styles.
If your 40AA 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 40AA 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 40AA is right for you.
40AA — frequently asked questions
What measurements are a 40AA bra in inches and cm?
A 40AA bra fits an underbust (ribcage) measurement of roughly 39 to 41 inches (99–104 cm) and a full-bust measurement of about 41 inches (104 cm). The cup letter AA reflects the roughly 1-inch (3 cm) difference between those two numbers.
What size is equivalent to 40AA? (sister sizes)
The sister sizes of 40AA are 38A (tighter band, one cup letter up). They hold the same cup volume as 40AA on a different band. Try a sister size when the cups of your 40AA fit but the band feels too loose or too tight.
What is 40AA in UK, EU, and other sizing systems?
40AA (US) converts to 40AA in UK sizing, 90AA in EU sizing, 105AA in French sizing, 18AA in Australian sizing, and 90AA in Japanese sizing. The body is the same; only the labelling system changes.
Is 40AA a small or large bra size?
In terms of how common it is, this size pairs an above-average band with a smaller 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 compact end.
Is 40AA 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 40AA 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 40AA?
Keeping the same 40 band, one size up in the cup is 40A. If you want a bigger or smaller band but the same cup volume, use a sister size instead: 38A. Remember that the cup letter is relative to the band — 42AA has more cup volume than 40AA, not the same.
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