A 40C is a 40 band with a C cup: an underbust of about 35 to 36 inches, and a bust two to three inches bigger. It is a wide-band size with a middle letter, and most of the decisions worth making at this size are band decisions wearing cup-letter costumes.
Not sure it is your size? The bra size calculator turns two measurements into an answer, sister sizes included.
What a 40C actually measures
The band is your underbust rounded up to an even number, plus four inches. The letter is the difference between bust and band: A at zero, one letter per inch, so C is two to three.
| Inches | Centimetres | |
|---|---|---|
| Underbust | 35 to 36 | 89 to 91 |
| Bust | 42 to 43 | 107 to 109 |
| Difference | 2 to 3 | 5 to 8 |
On UK and US charts, which use the underbust itself as the band, the same body is a 36E and a US 36DDD; EU charts call it a 80F. The bra size converter lines up all four systems.
Sister sizes: one real, one on paper
Move the band a step and the letter the opposite way, and the cup volume stays roughly the same:
42B ← 40C → 38D
Those two directions are not equally available. 38D is a stocked, ordinary size, ours included. 42B is honest arithmetic on a band Indian retail rarely makes, so for practical purposes the swap runs one way.
- Band rides up or feels slack, cup fits: go down to 38D. This is the common case, and the easy one.
- Band digs in, cup fits: the maths says 42B, and the shops mostly cannot help. Before hunting for it, re-measure; a digging 40 often turns out to be a cup problem pushing the band around.
- Band comfortable, cup spilling: stay on 40 and go up to 40D.
The general mechanics are in the sister sizes guide. How the same trade goes at A, B and D on this band is in the 40 bra size guide.
Let the band make the decision
At wider bands the band carries nearly all of a bra's support, so its behaviour outranks the letter. A band that fits sits level front to back, feels snug on the loosest hook when new, and admits two flat fingers underneath. Run that test before any size swap:
- Passes the test, cup wrong: change the letter, not the band.
- Rides up at the back: the band is too big, whatever the label says. 38D is the move.
- Leaves deep marks and relief when removed: too small, and no letter change will soften it.
The reason this ordering matters at 40C specifically: the size sits between a very available sister below and an unavailable one above, so drifting downward is frictionless and drifting upward is blocked. Make the move because the band test says so, not because the rail does.
Is a 40C big?
The question mixes two measurements, so it has two answers. The band is wide: a 35-to-36 inch underbust sits toward the upper end of the standard range, and that is a fact about the ribcage, not the bust. The cup is a middle letter: two to three inches of difference, the same C anyone wears.
What trips people up is that the same letter holds more volume as the band widens. A 40C contains noticeably more than a 36C, because each letter is measured against its own band. So "is a C big" has no answer at all until the band is attached, and comparing your letter with someone else's tells neither of you anything. The size describes a pairing, and only the pairing.
Backless outfits at 40C
Our strapless adhesive bra is made in 40C, so the size the calculator gives you maps to a real variant. No band, no straps: moulded cups adhere to clean dry skin, with the front clasp shaping the centre. At a wider band and a full cup the adhesive works harder, so preparation does more of the job here: clean, completely dry skin, no moisturiser that day, and firm pressure along every edge.
The short version
A 40C fits a 35-to-36 inch underbust with a bust two to three inches larger. Its buyable sister is 38D and its theoretical one is 42B, so band feel decides: slack or riding means 38D, digging means re-measure before anything else, spilling means 40D on the same band. The full method is in the bra size chart for India, and the letter below this one on the same band is covered in 40B.
