A bra size is two measurements, not one: the band size in inches around your ribcage, and the cup letter, which represents the difference between your underbust and your fullest bust measurement. Every bra size chart is doing the same arithmetic. The confusion comes from the fact that the same letter means different volumes in the US, the UK and the EU, and because brands add inches to the underbust that should not be added.
Below are the conversion tables, the difference table that turns measurements into a cup letter, and the traps that make people buy the wrong size three times in a row.
Band size: US, UK and EU
Band sizing is the most consistent part. US and UK band numbers are the same in inches. EU band sizes are in centimetres and are the inch size multiplied by 2.5, rounded.
| US / UK band | EU band | Underbust, inches | Underbust, cm |
|---|---|---|---|
| 30 | 65 | 28 to 29 | 71 to 74 |
| 32 | 70 | 30 to 31 | 76 to 79 |
| 34 | 75 | 32 to 33 | 81 to 84 |
| 36 | 80 | 34 to 35 | 86 to 89 |
| 38 | 85 | 36 to 37 | 91 to 94 |
| 40 | 90 | 38 to 39 | 96 to 99 |
| 42 | 95 | 40 to 41 | 101 to 104 |
| 44 | 100 | 42 to 43 | 106 to 109 |
| 46 | 105 | 44 to 45 | 111 to 114 |
Cup size: the difference table
Measure your fullest bust, subtract your underbust, and read across. This table is the one most US brands use.
| Difference, inches | US cup | UK cup | EU cup |
|---|---|---|---|
| Under 1 | AA | AA | AA |
| 1 | A | A | A |
| 2 | B | B | B |
| 3 | C | C | C |
| 4 | D | D | D |
| 5 | DD or E | DD | E |
| 6 | DDD or F | E | F |
| 7 | G | F | G |
| 8 | H | FF | H |
| 9 | I | G | I |
| 10 | J | GG | J |
The place this goes wrong is between DD and G. A US DDD, a UK E and an EU F are the same volume. A US G and a UK F are the same volume. If you order a UK brand using a US letter you will usually end up one cup too large.
The three traps that cause the wrong size
Trap 1: adding four inches to the underbust
Older sizing methods tell you to add four or five inches to your underbust measurement to get the band size. That method dates from when bras had little or no stretch. Modern bras use elastic fabric, so the band size should be your actual underbust measurement rounded to the nearest even number. If you measure 33 inches, you are a 34 band, not a 38.
Trap 2: assuming the same letter is the same volume
A 34D and a 38D are not the same cup. The letter describes a ratio, not a fixed volume. A 38D holds considerably more than a 34D. This is why a bra can fit in one band size and not in another at the same letter.
Trap 3: staying between sizes instead of moving two
If a 36C is slightly too small in the cup and slightly too loose in the band, the answer is not a 36D. It is a 34DD. Going down a band and up a cup keeps the cup volume roughly constant while tightening the band, and it is the single most useful adjustment in bra fitting.
Sister sizes
Bra sizes that hold the same cup volume at different band sizes are called sister sizes. If your size is not available, move one step in each direction:
| If your size is | The sister size up in the band | The sister size down in the band |
|---|---|---|
| 32D | 34C | 30DD |
| 34C | 36B | 32D |
| 36DD | 38D | 34E |
| 38DDD | 40DD | 36G |
Measure it properly once
Two numbers are all you need, and both are easier to take with a soft tape than a rigid one. Our step by step guide to measuring your bra size at home covers how to hold the tape, what to wear, and when to measure relative to your cycle.
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What to measure with
GDMINLO Soft Tape Measure, Double Scale, 60 inch / 150 cm
Brand: GDMINLO
Best for: the simplest option
A soft vinyl tape with inches on one side and centimetres on the other, which is exactly what both tables above need. Very high review volume for a low-cost item, and the flexible body is the reason it works around a ribcage where a rigid tape does not.
Rated 4.8 out of 5 from 67,565 ratings on Amazon, checked 9 November 2026.
iBayam 2-Pack Body Measuring Tape, 60 inch
Brand: iBayam
Best for: a spare to keep in the bathroom
Two tapes, which matters more than it sounds: the one you can find is the one you will use, and remeasuring every six months is the habit that keeps your size accurate. Same double scale, same flexible body.
Rated 4.8 out of 5 from 53,206 ratings on Amazon, checked 9 November 2026.
When to remeasure
- Every six months as a baseline, because band size drifts slowly and imperceptibly.
- After any change of more than about five pounds.
- After pregnancy, and again around six weeks after nursing ends.
- When a bra that used to fit needs the tightest hook to feel right.
- If you have never measured and have been wearing the same size since school.
If the numbers say one thing and the bra still feels wrong, the problem is usually the shape of the cup rather than the size. Our guides to cups that gap and bands that ride up cover what to change next.
Common questions
Why is my size different in every brand?
Because there is no enforced standard. Some brands use the difference table above, some use a variation of it, and some grade their cups more generously. Measure once, then treat the first purchase from a new brand as a test rather than a certainty.
Is 36C the same as 34D?
Not quite. They are close in volume but not identical, and the band difference changes how the bra sits. A 34D holds slightly more in the cup than a 36C on a smaller band.
What if my measurements land between two sizes?
Round the band to the nearest even number and try both cup sizes. Most people find one fits noticeably better once they are in the right band.
Do I measure over or under my clothes?
Over a thin, unpadded layer is fine. Never measure over a padded bra, since the padding will be included in the bust measurement and inflate your cup size.
The short version
Band size is your snug underbust in inches, rounded to the nearest even number. Cup size is the difference between bust and underbust, and the letter changes meaning between the US, the UK and the EU. If a size is close but not right, move one band size and one cup letter in opposite directions rather than changing only the cup.
