A 40A sits at the wide edge of what Indian shops stock, with the smallest difference the chart measures: an underbust of 35 to 36 inches and a bust at most an inch bigger. The combination is unremarkable on a body, nearly absent from shelves, and it comes with exactly one sister size.
Not sure it is your size? The bra size calculator turns two measurements into an answer, sister sizes included.
What does 40A measure?
| Inches | Centimetres | |
|---|---|---|
| Underbust | 35 to 36 | 89 to 91 |
| Bust | 40 to 41 | 102 to 104 |
| Difference | 0 to 1 | 0 to 3 |
The band is the underbust rounded up to an even number, plus four. The letter is the difference between the bust measurement and that band number, and at 40A the difference is zero to one inch. Take both figures separately, over bare skin with a level tape, and measure twice if the first number surprises you: at this end of the chart a single inch of underbust moves the band from 40 to 42.
On UK and US charts, which use the underbust itself as the band, the same body is a 36D; EU charts call it a 80D. The bra size converter lines up all four systems.
How can a 40 band be an A cup?
Because the letter never measured the bust. It measures the gap between bust and band, and a wide ribcage with a small gap produces exactly this label. In absolute terms the bust involved is not small: a 40A measures 40 to 41 inches around, more than a 32D's bust of 35 to 36. The A only sounds minimal if you read letters as volumes. They are differences, and differences do not translate across bands.
Why is 40A hard to find?
Ranges that run out to a 40 band usually run out to it in C and D, because labels pair wide bands with deep cups when they plan production. A wide band with a shallow cup falls between the plans. That is stocking logic rather than anatomy, and it is the same logic that empties shelves at the chart's other corner, where narrow bands meet deep cups.
The one sister: 38B
A sits at the bottom of the letter scale, so the usual two-way swap collapses to one:
40A → 38B
Nothing goes on the left of that arrow: a band-up sister would need a letter below A, and none exists. 38B is the entire list. One band down, one letter up, a similar cup volume on a firmer band, and it is stocked. Going a second step to 36C doubles the band cost for the same volume and is rarely worth it. The one-way arithmetic for A cups is covered in the sister sizes guide.
- Band rides up or shifts when you move: 38B is the swap, and the only one.
- Band digs in: no swap exists in that direction. Change the band itself, with an extender or a softer 40, because the size above with a smaller letter is not on any chart.
- Cups wrinkle or gape while the band fits: try a shallower cut in the same size before touching the numbers. A zero to one inch difference will not fill a deeply moulded cup, and no size change fixes a style problem.
The same shallow difference two bands narrower is 38A bra size, which faces the same one-sister arithmetic with its own numbers. The 40 bra size guide widens the view past the A and shows where each letter's sisters run out.
Backless and strapless outfits at 40A
The band does nearly all the work at this width, so outfits that delete it need adhesion in its place. Our strapless adhesive bra is moulded in 40A, the last A on its grid, and holds by silicone contact with a front clasp. At a shallow difference it is mostly supplying shape and coverage rather than carrying weight, which is the easiest job adhesion is ever asked to do. Clean, dry skin and a fully pressed edge still apply.
The short version
A 40A is an underbust of 35 to 36 inches with a bust of 40 to 41: a wide band, a shallow difference, one sister. If the band is loose, 38B is the entire menu of swaps. If the band is tight, no swap exists and the band itself has to change. The chart and the method behind it are in the bra size chart for India.
