Bra Size Converter

Two band methods, two sizes apart

The band comes from the underbust measurement alone, and there are two ways to turn one into the other.

The traditional method adds four inches to the underbust before rounding — a convention from an era of far less elastic fabric, when a band had to be looser to be wearable at all. The modern method adds nothing, because elastic does the work now.

Four inches is two full band sizes, at every measurement — the gap is constant. So the same person is a 30 band or a 34 depending only on which chart they happened to find, and most charts do not say which method they used.

The European convention sidesteps it entirely: an EU band is the underbust in centimetres, rounded to the nearest five. A 75 band means a 75 cm ribcage, with nothing added and nothing to guess.

Which method to use in practice: most contemporary fitters and most brands designing for modern elastic use the no-addition method, and a band that rides up at the back is the classic sign of one that is too large. But a brand’s own size guide is the authority for that brand.

A UK G is not a US G

The cup letter comes from the difference between the bust and underbust measurements — one letter per inch. Both systems agree on that, and both agree on the letters, right up to DD. Then they separate.

The British scale doubles letters — DD, FF, GG — where the American one adds new ones, so the two run out of step from six inches onward. A US G is a seven-inch difference and a UK G is a nine-inch one: two cup sizes apart under the same letter.

This is why ordering from a British retailer using an American size, or the reverse, goes wrong specifically in the larger cups and not at all in the smaller ones. Below DD there is nothing to convert; above it, the letter alone does not identify anything.

Australian sizing follows the UK letters with a different band numbering again, and some brands print both. When a listing gives two letters, that is usually what it is doing.

A cup letter is not a volume

This is the most useful thing on the page, and the least well known. A cup letter is a difference, not a size — so the same letter means different volumes on different bands.

A 34C has a three-inch difference on a 34-inch band. Take the band down two inches and the difference for the same bust becomes four, so it is a 32D. Take it up and it becomes 36B. All three hold roughly the same cup:

Two consequences follow. “I’m a C” is not a statement about a person — it describes a pairing, and the same body is a C, a D or a B depending on the band. And when a band feels loose but the cup fits, the fix is not a smaller everything: it is one band down and one cup up, which keeps the cup and tightens the band.

It is also why band-size complaints are so common. A band does most of the support, so a band that is too large moves the work to the straps, which is what produces shoulder marks — and the instinct to buy a smaller cup makes it worse rather than better.

Measuring, and how much it moves

Two measurements, both with a soft tape, both level all the way round — check in a mirror, because a tape that rides up at the back reads short.

The underbust goes directly beneath the bust, snug against the ribcage. Breathe out normally; a measurement taken on a full breath will be a size out. The bust goes around the fullest point with the tape loose enough not to compress anything.

Now the part worth knowing. One cup size is one inch of difference — just 2.54 cm. And an ordinary measurement moves by around 2.5 cm through a day and across a cycle: with breathing, posture, temperature, and hormonal changes. That is essentially a whole cup size of natural variation.

So a single reading is a snapshot, not a fact. Measure more than once, on different days, and treat the result as the first size to try rather than the size you are — which is the same conclusion this site reaches about shoes and rings, and for the same reason: the scale is finer than the body is stable.

Why the size still varies by brand

Even with both measurements right and the method settled, the same nominal size fits differently between makers. That is not a failure of the arithmetic; it is that the arithmetic only describes two circumferences.

There is no international standard for bra sizing. Each brand works from its own block — its own assumptions about shape, projection, wire width, gore height and how much the fabric gives — and two garments labelled 34C built on different blocks are genuinely different objects.

Style matters as much. A balconette, a plunge and a full-cup in the same nominal size hold different amounts and sit differently, because the cup shape is different rather than the cup volume. Fabric matters too: a stretch band and a rigid one at the same nominal size are not interchangeable.

What follows is practical rather than defeatist. Use a calculated size to pick a starting point and a sister size or two to try alongside it. Judge the fit by what it does — the band level and firm, the wire lying flat against the ribcage, the cup containing without gaping — rather than by the label. And when a brand publishes its own size guide, use it, because it knows its block and this page does not.

Related calculators

Other sizing systems that do not agree with each other:

More in Conversion, or browse all calculators.

Sources and methodology

There is no international standard for bra sizing, which is the root of everything on this page — the European approach of designating a size directly from a body measurement in centimetres is standardised, and the British and American letter scales are trade conventions that grew separately. So the sources establish the measurement definitions and the metric convention, and the divergences below are documented industry practice rather than anything a standards body settled.

• EN 13402 — Size designation of clothes: body measurements and intervalsEuropean Committee for Standardization (CEN) · verified 2026-09-07 · The European convention of designating sizes from body measurements in centimetres, which is why the EU band is the underbust measurement itself rather than a derived number
• ISO 8559 — Size designation of clothes: garment construction and anthropometric surveysInternational Organization for Standardization · verified 2026-09-07 · The definitions of the underbust and bust measurements used here, and the principle that a size designation is derived from body measurements rather than from garment dimensions
• NIST Handbook 44, Appendix C — the international inch as exactly 25.4 mmNational Institute of Standards and Technology, US · verified 2026-09-07 · The inch-to-centimetre factor behind every band and cup step on this page — one cup is one inch of difference, which is 2.54 cm exactly