A 40B fits an underbust of 35 to 36 inches with a bust one to two inches larger. From 38 up, the fitting rules tilt: the cup letter matters less and less, and how the band sits decides nearly everything about how the bra behaves.
Not sure it is your size? The bra size calculator turns two measurements into an answer, sister sizes included.
The measurements behind 40B
The band is the underbust rounded up to an even number plus four, so 35 to 36 inches becomes a 40. The letter is the bust measurement minus the band number, and one to two inches of difference is a B. Measure over bare skin, tape level, nothing padded under it.
| Inches | Centimetres | |
|---|---|---|
| Underbust | 35 to 36 | 89 to 91 |
| Bust | 41 to 42 | 104 to 107 |
| Difference | 1 to 2 | 3 to 5 |
On UK and US charts, which use the underbust itself as the band, the same body is a 36DD; EU charts call it a 80E. The bra size converter lines up all four systems.
Is 40B a big bra size?
The question folds two measurements into one word, and they answer differently. The 40 records a broad ribcage. The B records a modest one to two inch difference. Together they describe a wide frame carrying a shallow bust, which no single adjective covers. The same letter on a narrower band belongs to a visibly different body: a 38B carries the identical difference on an underbust two inches smaller.
How should a 40 band fit?
The band should sit parallel to the floor all the way round, stay put when you raise your arms, and hold its line without cutting in. A band that climbs up the back is too big, whatever the label says. One that leaves deep marks and shortens your breath is too small. In between is a firmness you stop noticing by mid-morning.
At this width the band carries nearly all the support and the straps only steady the cups, so read every complaint as a band complaint first. Slipping straps and bounce usually trace to a band that has stretched, not to anything the cups are doing. Elastic relaxes over months of wear, which is why a band that fit when new rides up half a year later: move to the tighter hooks as it goes, and treat running out of hooks as the sign to act.
What is the sister size of 40B?
42A ← 40B → 38C
Band up on the left, band down on the right, cup volume roughly constant across the three. The left option is the honest disappointment: 42A is correct arithmetic, and Indian rails rarely run past 40, so it is close to unbuyable. The right option is stocked. The pairing rules live in the sister sizes guide.
- Band rides up while the cup fits: go down to 38C, and expect the extra firmness to feel like the point rather than a penalty.
- Band digs in while the cup fits: 42A is the paper answer; in practice a band extender on your 40, or a 40 cut with softer elastic, settles it sooner than the hunt will.
- Cup spills while the band sits well: stay on the band and take 40C.
The 40 bra size guide does this accounting for all four letters at once.
Strapless and backless outfits at 40B
An outfit that removes the band removes the thing doing the work, which is why strapless dressing gets harder as bands widen. Adhesion is the replacement: our strapless adhesive bra is moulded in 40B, sticks to clean dry skin, and clasps at the front, so there is no band to ride up or dig in at all. What it asks in exchange is a proper application. Skin has to be freshly cleaned and fully dry, with no moisturiser anywhere the silicone lands, and the whole edge pressed down, because hold comes from contact area and every skipped stretch of edge is hold given away.
The short version
A 40B is an underbust of 35 to 36 inches with a bust of 41 to 42. At this width the band is nearly the whole fit: level, firm and parallel to the floor, with every common complaint checked against it first. The sister size worth knowing is 38C, stocked and one band snugger; 42A mostly exists on paper in India. The method and the full table are in the bra size chart for India.
