Two measurements, and one piece of arithmetic that has not changed in decades. What has changed is the advice around it, and a lot of what is still printed on size charts is actively wrong. This page gives you your size, both sister sizes, and the international equivalents, and it explains the reasoning at every step so you can tell when a shop assistant is guessing.
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How to measure
Wear an unpadded bra, or none at all. A padded one adds volume that is not yours and throws the cup out by a full letter.
- Underbust: wrap the tape directly under your bust, snug and level all the way around, and breathe out normally. This should feel firm. It is the number that decides your band, and the band does most of the work.
- Bust: measure loosely around the fullest point, tape level, without compressing anything. Lean forward slightly if that helps the tissue settle naturally.
Measure each one twice. If two readings disagree, take them again rather than averaging, because a drifting tape usually means it was not level. No soft tape? Wrap a piece of string around the same line, mark where the end meets the string, then lay the string flat against a ruler.
How the calculation works
Your band size is your underbust measurement, rounded to the nearest even number. Your cup comes from the difference between your bust and your band: roughly one inch of difference per cup letter, so a four inch difference is a D.
If you have been told to add four or five inches to your underbust first, ignore it. That instruction dates from an era of stiff, non-stretch fabric that needed the slack designed in. Modern bands are elastic, and the old method sends most people several band sizes too loose. It is the single biggest reason bras ride up at the back.
The formula
cup = bust − band, at about one inch per letter
EU band = band × 2.5 − 10 · FR = EU + 15 · AU = band − 22
Underbust and bust are the two measurements above, in inches (centimeters are converted for you). The cup ladder runs AA, A, B, C, D, DD, DDD, G, H in US sizing, and the difference in inches gives you the position on that ladder.
One detail worth stating, because plenty of charts get it wrong: nearest even really does mean nearest. An underbust of 32.5 is half an inch from a 32 band and an inch and a half from a 34, so it is a 32. And when a measurement lands exactly between two even bands, we give you the snugger one, because bands stretch with wear and a bra should start life on its loosest hook.
Worked example
Sister sizes: the most useful bra fact nobody explains
Cup letters are not absolute volumes. A D cup on a 32 band is smaller than a D cup on a 38 band, because the cup is scaled to the band it sits on. Which means the same volume of breast tissue appears under different letters depending on the band: a 34D, a 32DD and a 36C all hold about the same amount.
This is genuinely practical. If a bra's cups fit you well but the band digs in, you do not need a different bra, you need the same volume on a bigger band: go up a band and down a letter. If the band rides up your back, go down a band and up a letter. Most people who believe they are an unusual size are simply one sister size away from a shelf full of options.
International conversion
Band numbers change in every country, and above a DD cup the letters diverge too. Match by position on the ladder, never by the letter alone.
| US band | UK | EU | FR / ES | IT | AU |
|---|---|---|---|---|---|
| 28 | 28 | 60 | 75 | 0 | 6 |
| 30 | 30 | 65 | 80 | 1 | 8 |
| 32 | 32 | 70 | 85 | 2 | 10 |
| 34 | 34 | 75 | 90 | 3 | 12 |
| 36 | 36 | 80 | 95 | 4 | 14 |
| 38 | 38 | 85 | 100 | 5 | 16 |
| 40 | 40 | 90 | 105 | 6 | 18 |
| 42 | 42 | 95 | 110 | 7 | 20 |
| 44 | 44 | 100 | 115 | 8 | 22 |
| US cup | UK cup | EU / FR / IT cup |
|---|---|---|
| A | A | A |
| B | B | B |
| C | C | C |
| D | D | D |
| DD | DD | E |
| DDD | E | F |
| G | F | G |
| H | FF | H |
When the fit is wrong, what to change
The band does around 80 percent of the work of holding a bra up. The straps are mostly for shaping, which is why a loose band shifts the load to your shoulders and makes them ache. A correct band sits level around your body and lets you slide two fingers underneath, no more. Buy so it fits on the loosest hook, because bands stretch with wear and the tighter hooks are the life of the garment.
| What you notice | What it usually means |
|---|---|
| Band rides up at the back | Band too loose. Go down a band, up a cup letter. |
| Band digs in or leaves marks | Band too tight. Go up a band, down a cup letter. |
| Tissue spilling over the top or side | Cup too small. Go up a cup letter, same band. |
| Cups gape or wrinkle | Cup too big, or the style is the wrong shape for you. Try a cup down before changing style. |
| Underwire sits on breast tissue | Cup too small, or the wire is too narrow for how far apart your breasts sit. |
| Straps dig into shoulders | Almost always the band is too loose, not the straps too tight. Fix the band first. |
| Center panel does not lie flat | Cup too small, or the style does not match your breast shape. |
Why brands still will not agree
There is no enforced standard for bra sizing anywhere in the world. Cup volume at a given letter varies with the style, the fabric, and how much stretch the brand builds in, so a size that fits beautifully in one label can be a full cup out in another. That is normal and it is not a fault in your measuring.
Which is the honest limit of this page, and of any page like it. Two numbers cannot capture breast shape, tissue firmness, or how far apart your breasts sit, all of which decide which style suits you at the same size. Treat the result here as the size to take into the fitting room first, and carry your sister sizes with you.