Bra Size Calculator

cup difference = bust − underbust, then band = the nearest 5 cm (metric) or nearest even inch (US), and cup = the letter whose nominal difference is closest to that difference

underbust

Measurement around the ribcage directly under the bust — this sets the band number

bust

Measurement around the fullest point of the bust, tape level

difference

Bust minus underbust. This is the only number that decides the cup letter

Use it when you have measurements in centimetres and need a size to shop for, or when you are converting between a metric size and a US one and want to know why the letters do not line up. The cup letter follows from the difference alone, so it is worth knowing that a size that gapes at the top and a size that digs in at the band are two different problems with two different fixes: the cup is a difference, the band is a length. What the arithmetic cannot tell you is whether a particular style will fit, because cup depth and shape vary between manufacturers even at the same nominal size.

1. 75 cm underbust, 87.5 cm bust, metric

   1. Difference: 87.5 − 75 = 12.5 cm
   2. Band: the underbust is already on the 75 cm mark of the 5 cm grid, so the band is 75
   3. Cup: 12.5 cm is exactly the nominal difference for a B in the metric ladder
   4. Size: 75B

   This is the measurement the next example converts, and the two results are the point of the page. Nothing about the body changed — only the system did.

2. The same measurements read as a US size

   1. Difference: still 12.5 cm, because it is a measurement and does not depend on the system
   2. Band: 75 cm ÷ 2.54 = 29.53 in, and the nearest even inch is 30
   3. Cup: 12.5 cm ÷ 2.54 = 4.92 in, and the nearest whole inch is 5 — the fifth letter above AA, which is DD
   4. Size: 30DD

   A 12.5 cm difference is a B in one system and a DD in the other. The difference sits between the 4-inch and 5-inch marks, and rounding to the nearer one is what makes it DD rather than D. Under the +4 method the same body would be labelled with a band about four inches larger and a smaller cup letter to go with it — see the FAQ.

These are nominal sizes, and nominal sizes are a starting point rather than a fit. The metric ladder is tabulated at exact points — 10 cm, 12.5 cm, 15 cm — and this page takes the nearest one, while many published charts print each nominal value as the start of a range, which shifts the answer by one cup in the upper half of every band. Cup names above D are not standardised in the US at all: DD/DDD, D1–D5 and D/DD/E/F are all in retail use, and this page follows the DD/DDD convention. Nothing here is a mandatory international standard — the cup layer in particular is set by national standards and retail convention rather than by any binding agreement, so a size label means slightly different things in different shops. The measurement itself is the largest source of error: a tape that is not level, or measured over a padded bra, moves the difference by more than a cup. Measure without a bra, keep the tape level and snug, and treat the result as a starting point for trying sizes on.

Why is a 75B a 30DD and not a 34B?

Because the two ladders start in different places. The metric cup ladder starts at 10 cm for an A and steps by 2.5 cm, so 12.5 cm is a B. The US ladder starts at one inch for an A and steps by one inch, so 12.5 cm — 4.92 inches — is five steps above AA, which is a DD. The steps are almost the same size, so the letters look like they should line up; they are simply offset by about three. A 34B has a 34-inch band, and this measurement is a 30-inch band, so the number is wrong too.

What is the +4 rule, and why does this page not use it?

Under +4 you add four inches to the underbust before choosing the band: 29.53 inches rounds to 30, and four more makes 34. The cup is then taken against that inflated band rather than against your ribcage, so the letter comes out smaller as well — the two halves of the size move together, which is why a +4 chart and a +0 chart agree on neither the number nor the letter. The method dates from an era of non-stretch fabric, when a band needed the extra room, and it is still what many US retailers print. This page uses +0, which reports the measurement as taken. Both are in use, which is exactly why a size label is not portable between shops: if a US brand's band fits you about four inches looser than this page suggests, that brand is using +4.

How do I measure the underbust and the bust?

Measure without a bra and without compressing the tissue. For the underbust, run the tape around the ribcage directly beneath the bust, keep it level all the way round and pull it snug — this one should be firm, because it is the measurement that carries the weight. For the bust, run the tape around the fullest point, usually over the nipple, and keep it level and relaxed rather than tight. Breathe out normally for the underbust. If the two measurements come out within a centimetre or two of each other the cup is very small; that is a real result, not an error.

Is there an international bra size standard?

Not one that binds manufacturers. EN 13402 defines the vocabulary — the cup is the difference between the bust and the underbust — but it is voluntarily adopted, and the cup ladder itself is set nationally or by retail convention. China's FZ/T 73012 gives an explicit cup table and a 5 cm band grid, which is why the metric side of this page has a definite answer where the US side is a convention. Internationally, a printed size is a label a brand chooses, not a measurement it obeys.

Does the size change if I lose or gain weight?

Usually the band changes first, because the ribcage carries most of the change, and the cup letter often moves in the opposite direction — a smaller band with the same bust is a larger cup. That is why re-measuring matters more than remembering a size: a 80B and a 75C can be nearly the same body, and the same cup letter at two bands is not the same volume. If the difference between your measurements comes out the same after a change in weight, expect the cup letter to hold and the band number to move.

• Bra size — the metric and US systems, the +4 convention and its history — Wikipedia
• EN 13402 — Size designation of clothes, including the cup definition that the metric ladder follows — Wikipedia