A bra size is two measurements and one subtraction.
The part nobody explains is that an Indian band size is not your underbust measurement. It's that measurement plus four inches. A 34 band is built for someone who measures about 30 inches underneath. Miss that and every number you calculate will be four sizes out.
Prefer not to do the arithmetic? Our bra size calculator does all of this, including sister sizes, in one step.
The two measurements
You need a soft measuring tape and a mirror. Wear a thin, non-padded bra or nothing on top.
1. Underbust. Measure directly under your bust, where the band sits. Pull the tape snug: firm enough that it won't slide down, not so tight it digs in. Breathe out and take the reading.
2. Bust. Measure around the fullest part of your bust, tape level all the way round and not pulled tight. Stand normally; don't lean forward.
Your band is your underbust plus four
This is the Indian convention, and it's what the brands you're shopping from use. Round your underbust up to the nearest even number, then add four.
| Underbust | Band size | |
|---|---|---|
| 23–24 in | 58–61 cm | 28 |
| 25–26 in | 64–66 cm | 30 |
| 27–28 in | 69–71 cm | 32 |
| 29–30 in | 74–76 cm | 34 |
| 31–32 in | 79–81 cm | 36 |
| 33–34 in | 84–86 cm | 38 |
| 35–36 in | 89–91 cm | 40 |
| 37–38 in | 94–97 cm | 42 |
| 39–40 in | 99–102 cm | 44 |
It looks like an error the first time you see it. It isn't. The number on the label is a label, not a measurement of your ribcage, and the garment is cut to match it. A 34 band from an Indian brand is made to fit a 30-inch underbust, so it does.
Your cup is the difference
Subtract your band from your bust. Each inch of difference is one cup letter, starting at A for no difference at all.
| Bust minus band | Cup |
|---|---|
| 0–1 in | A |
| 1–2 in | B |
| 2–3 in | C |
| 3–4 in | D |
| 4–5 in | E |
| 5–6 in | F |
Note there is no DD in this system. It runs A, B, C, D, E, F.
Worked example. Underbust 30 inches, bust 37 inches.
- 30 is already even, so 30 + 4 = band 34
- 37 − 34 = 3 inches of difference = D cup
- Your size: 34D
A second one. Underbust 40, bust 48. The band is 44. The difference is 4 inches, so an E cup. 44E.
Landed exactly on a boundary: the difference is precisely 3 inches? Round up to the larger cup. A cup that's slightly generous can be managed; one that's slightly small can't be fixed at all.
Why your size looks different on international sites
Because the UK and US don't add the four inches. There, your band size is your underbust measurement, and cup letters start at one inch of difference rather than zero.
Same body, different labels:
| Underbust | Bust | India | UK |
|---|---|---|---|
| 30 in | 37 in | 34D | 30F |
| 32 in | 38 in | 36C | 32E |
| 40 in | 48 in | 44E | 40FF |
Neither is wrong. They're different labelling conventions for the same garment, much as shoe sizes differ between countries. What matters is using the convention of whoever you're buying from, which is why a size that fits perfectly here can look alarming on a British site, and the reverse.
The UK system also has a DD between D and E, and the US splits again above that into DDD. If you're comparing across sites and the letters stop lining up, that's why.
Sister sizes
Every size has neighbours holding roughly the same cup volume on a different band. Move the band one step and move the cup letter the opposite way:
36C ← 34D → 32E
All three hold a similar volume. They differ in how tightly the band sits.
Useful in practice: if the cup fits but the band feels loose, go down a band and up a cup. If the band is right but you're spilling over, keep the band and go up a cup letter only.
The four questions people actually ask
Which cup is bigger: B, C or D? D, then C, then B: but only at the same band size. A cup letter isn't a fixed volume. It's the gap between your bust and your band, so a C on a 42 band holds considerably more than a C on a 30 band.
Is 34 or 36 a bigger bra size? 36 has the bigger band. On Indian sizing that number is your underbust plus four, so a 36 suits roughly a 31–32 inch underbust and a 34 suits 29–30.
Is 34C or 36C bigger? 36C. Same letter, larger band, and the cup scales up with the band, so a 36C holds roughly what a 34D does.
How do I calculate my bra size? Round your underbust up to an even number and add four for the band. Subtract the band from your bust, and each inch is one cup letter starting at A. That's the whole calculation.
Why the same size fits differently across brands
Because there's no enforced standard. Cup depth, wire width, strap placement and how much the band stretches all vary between manufacturers, and Indian brands draw patterns from several different traditions.
Treat your calculated size as a starting point, not a verdict. Then check the fit:
- The band rides up your back → too loose, go down a band
- Straps dig in or leave marks → the band isn't carrying the weight
- Tissue spills over the top or sides → cup too small
- The cup gapes at the top → cup too big, or the wrong shape for you
- The centre panel doesn't sit flat against your breastbone → cup too small
A correctly fitted band should be snug on the loosest hook, leaving the tighter hooks for when it stretches over months of wear. If a bra only fits on the tightest hook when it's new, it's already too big.
More on which style suits which shape in our guide to bra types, and on what fit can and can't change in the best bra for sagging breasts.
When no size on the chart is the answer
Sometimes the measurement isn't the problem: the outfit is. A backless dress has nowhere for a band to sit, and a strapless neckline removes the shoulder anchor, which is why strapless bras slip most for the people who most need them not to.
Your measured size still matters there. Our silicone strapless adhesive bra uses this same band-and-cup system, so the size you just worked out is the size to order. It has no band and no straps at all: the cups adhere to skin, so support comes from adhesion rather than tension.
For a deep back or a very open cut, lift tape sized by cup does the same job with more flexibility. And if your strapless bra slides today, that's usually band size rather than the bra: worth re-measuring before replacing it.
The short version
Measure your underbust, round up to an even number, add four: that's your band. Measure your bust, subtract the band, and each inch of difference is one cup letter starting at A. There's no DD. If you're comparing against a UK or US site, expect a smaller band number and a larger letter for the same body.
Then try it on, because the chart gets you to the right starting point and the fit check tells you the rest.
General information, not medical advice. Measurements are a guide: fit varies between brands and bodies.
