40B Bra Size

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 two inches larger than the underbust — most cup shapes are widely available at this volume.

40B in Other Sizing Systems

EU and FR sizing read the band in centimetres, which is why 40B maps to 90B and 105B respectively — 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:

Not sure about your size? Measure yourself and double-check.