A cup size is the difference between two numbers: your bust measurement and your band size. Each letter stands for one more inch of difference, starting at A. So a cup letter is not a fixed volume, and it tells you very little until you read it with the band number in front of it.
What does cup size mean?
On Indian sizing, the band is your underbust rounded up to an even number, plus four. The cup is your bust minus that band.
| Bust minus band | In cm | Cup |
|---|---|---|
| 0 to 1 inch | 0 to 2.5 | A |
| 1 to 2 inches | 2.5 to 5 | B |
| 2 to 3 inches | 5 to 7.5 | C |
| 3 to 4 inches | 7.5 to 10 | D |
| 4 to 5 inches | 10 to 13 | E |
| 5 to 6 inches | 13 to 15 | F |
A difference that falls exactly on a line takes the larger letter, so two inches is a C. This is the same rule the bra size calculator applies.
How do you measure cup size?
You need two measurements and one subtraction.
- Underbust. Wrap the tape around your ribcage directly under the breasts, snug and level, and breathe out. Say it reads 29.5 inches.
- Bust. Wrap the tape around the fullest part, level front to back, resting on the body without pulling. Say it reads 36.5.
- Band. Round the underbust up to the next even number and add four. 29.5 becomes 30, and the band is 34.
- Cup. Subtract the band from the bust. 36.5 minus 34 is 2.5, which falls in the two to three inch row. That is a C.
The size is 34C. If your tape is in centimetres, the calculator takes them directly, and the working is in what underbust and overbust mean.
The mistake to avoid is subtracting the underbust from the bust. That gives a number four to six inches too big and a letter several steps too high. The cup is measured from the band.
What is the difference between A, B, C and D cups?
On the same band, one inch of bust each.
| Size | Underbust (inches) | Bust (inches) |
|---|---|---|
| 34A | 29 to 30 | 34 to 35 |
| 34B | 29 to 30 | 35 to 36 |
| 34C | 29 to 30 | 36 to 37 |
| 34D | 29 to 30 | 37 to 38 |
So a 34B and a 34C share a ribcage, and the C has about an inch more at the fullest point. That is the whole difference between a B cup and a C cup, or between any two neighbouring letters.
Each letter has its own page with the measurements for every band: A cup, B cup, C cup, D cup, and E and F.
Is a C cup the same size on every band?
No. The letter is a difference, so the cup it describes grows with the band. A 38C holds more than a 34C, which holds more than a 30C.
It also means different sizes can share a cup volume. Go up one band and down one letter, or the other way, and the cup holds about the same: 32C, 34B and 36A are close to each other in volume and differ in how long the band is. These are called sister sizes, and the sister size guide explains when swapping to one makes sense.
This is why "is a C cup big?" has no answer. A 30C is a small bra. A 40C is not. The question only works with the band attached.
Does bust size tell you your cup?
Not by itself. A 36 inch bust can be a 32E, a 34C or a 36A, depending on the ribcage under it. What bust size means has that example worked through.
Why is my cup letter different on a foreign label?
Because UK and US bands are smaller numbers. They use the underbust without adding four, and then the same bust sits further from the band, so the letter comes out higher. An Indian 34B is the same body as a UK 30DD. Nothing about you changed between the two labels. The bra size converter shows the equivalents.
Some Indian labels also print DD for the step after D. The chart above goes straight from D to E for that step.
Where does cup size matter for stick-on products?
More than the band does, for most of them. The butterfly stick-on bra is sized by cup alone, in A-B, C and D-D++. The fabric uplift cups are Small for an A cup, Medium for B to C and Large for D and above. Reusable nipple covers come in 8, 10 and 13 cm, which the product page maps to A, B to C and D-plus.
The strapless adhesive bra is the exception. It is sized like a bra, band and cup together, from 30A to 40D.
The short version
Cup size is bust minus band, one letter per inch, with A at no difference. The band is your underbust rounded up to even plus four, so measure both and subtract from the band, not from the underbust. Neighbouring letters are an inch apart on the same band, and the same letter is a bigger cup on a bigger band. Two measurements in the bra size calculator give you the band, the letter and the sister sizes together.
