40B Bra Size
40B fits a 39–41 inch (99–104 cm) underbust and a full bust of about 43 inches (109 cm). Its sister sizes are 38C and 42A.
Last reviewed on 25 September 2026.
40B at a glance
| Underbust (band) | 39–41 in · 99–104 cm |
|---|---|
| Full bust | ≈ 43 in · 109 cm |
| Bust minus underbust | ≈ 3 in · 8 cm |
| Sister sizes (same cup volume) | 38C · 42A |
| UK / EU / FR / AU | 40B · 90B · 105B · 18B |
| One cup bigger | 40C |
| One cup smaller | 40A |
| Approx. top size | XL · UK 16–18 / US 12–14 |
Measurements for 40B
40B is built for an underbust ribcage measurement that falls in the 39–41 inch range, with a full-bust measurement around 43 inches. The band number — 40 — is a larger band where reinforced construction starts to matter for long-day comfort. The cup letter — B — describes the difference between the two measurements; a B cup means the full bust is about three inches larger than the underbust — most cup shapes are widely available at this volume.
Shop 40B Bras
Confident 40B is your size? See what's available in it right now — sorted by style.
Shop All 40B Bras on AmazonAs an Amazon Associate, BraCalc earns from qualifying purchases — at no extra cost to you.
40B in Other Sizing Systems
EU, FR, and JP sizing read the band in centimetres, which is why 40B maps to 90B (EU), 105B (FR), and 90B (JP) — the same body, expressed in different units. AU sizing uses a small numeric scale (18B for this band). UK sizing shares the band number with the US but parts ways at the cup once you go past D, so 40B reads as 40B on a UK label.
Sister Sizes for 40B
These sizes have the same cup volume as 40B. 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 40B 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 40B
B cups are among the most widely produced sizes, so style choice is broad. If you see gapping at the top of the cup, a demi-cup or push-up shape often fits better than a full-cup.
If your 40B 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 40B 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 40B is right for you.
40B — frequently asked questions
What measurements are a 40B bra in inches and cm?
A 40B bra fits an underbust (ribcage) measurement of roughly 39 to 41 inches (99–104 cm) and a full-bust measurement of about 43 inches (109 cm). The cup letter B reflects the roughly 3-inch (8 cm) difference between those two numbers.
What size is equivalent to 40B? (sister sizes)
The sister sizes of 40B are 38C (tighter band, one cup letter up) and 42A (looser band, one cup letter down). They hold the same cup volume as 40B on a different band. Try a sister size when the cups of your 40B fit but the band feels too loose or too tight.
What is 40B in UK, EU, and other sizing systems?
40B (US) converts to 40B in UK sizing, 90B in EU sizing, 105B in French sizing, 18B in Australian sizing, and 90B in Japanese sizing. The body is the same; only the labelling system changes.
Is 40B 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 40B 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 40B 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 40B?
Keeping the same 40 band, one size up in the cup is 40C, and one size down is 40A. If you want a bigger or smaller band but the same cup volume, use a sister size instead: 38C or 42A. Remember that the cup letter is relative to the band — 42B has more cup volume than 40B, not the same.
Not sure about your size? Measure yourself and double-check.
Calculate My SizeAlready sure? Shop 40B bras on Amazon →