Bra size: band and cup from two measurements

Which two measurements you need

Underbust — the tape runs horizontally right under the bust, snug but not cutting in; this is where a bra's elastic band sits, and the band is what carries the weight. Bust — the tape runs across the fullest point, loose, without flattening the fabric or pulling it in. Stand straight while you do it, arms down, breathing normally.

Nothing else is needed: not height, not weight, not your clothing size. A bra size is arithmetic on two measurements and nothing else, and that is where it differs from a dress size, which is read from three numbers at once.

Band: your underbust, rounded to a multiple of five

The band ladder steps by 5 cm — the EN 13402-3 ladder, from 60 to 125. Your measurement lands on the nearest step: 72.5–77.5 cm is band 75. The left column of the table shows the interval rather than the nominal, because published charts print the row as 60-65-70, and someone who measures 73 does not find a row for it.

The UK and US number is that same measurement in inches plus four: 75 cm turns into 34. In effect this is the «+4» fitting method written into a conversion chart, and the metric cup letters carry the same four inches, which is why the two tables agree letter for letter. The «+0» method takes the band straight off the tape instead, so 73–77 cm is a 30, and counts the cup over that smaller band: an underbust of 75 cm with a bust of 92 cm is 34C by this table and 30E by +0. Published conversion tables carry both rows. The argument is about exactly this conversion; this page follows the +4 row because the common charts do. What that means in a fitting room is further down, in the section on sister sizes.

The French number is the European one plus fifteen: 75 is printed as 90. The Italian number counts the step rather than the measurement: 1 is 70 cm, 2 is 75, and on by fives — and the two lowest European bands have no Italian number at all. The table gives them a dash, not an invented zero.

Cup: how much bigger the bust is than the band

This is the point where the pages written on this subject part company, and they part company silently. Two rules are in circulation: the cup is counted either from the underbust the TAPE actually read, or from the NOMINAL value of the band that reading was rounded to. The gap between them reaches a full letter.

This table counts from the nominal, because that is how the published charts themselves are built. The RU and EU ones print a row for a band of 68–72 cm: AA at a bust of 80–82, A at 82–84, B at 84–86. That is 70 + 10–12, 70 + 12–14, 70 + 14–16 — nominal 70, not the 68 or 72 the reader actually measured. Under the first rule, someone with a 72 cm underbust and an 88 cm bust would get a C where every store's chart prints a D.

The top of each interval belongs to the next letter: exactly 12 cm is already A, not AA. Counting from the nominal costs one thing, and the cost is an honest one: right at a band boundary the answer jumps by a letter. Someone who measures 72.5 can legitimately read both band 70 and band 75 — and both give a real size, just a different one.

Why 75C, 70D and 80B are the same cup

Because the cup is measured OVER the band. Go up a step on the band, go down a letter on the cup, and the volume stays the same. This is not folklore and not a sales trick, it follows directly from the arithmetic above: 30D, 32C, 34B and 36A all hold the same volume.

In a store this is the single most useful thing to know. The printed size that actually fits is very often not the one the tape named — and the sizes next to it on this line are worth trying on before you decide the tape was wrong. The «+0 versus +4» argument is a different matter. Fitters who find the standard row too loose in the band are not pointing at the next size on this line: four inches is two bands, not one, and +0 counts the cup over the smaller band, so a 34C comes out at about 30E — more cup than 30DD, its sister two bands down. If the band rides up at the back and the cup overflows, that is the size to try.

The RU bra size is the EU one

There is no separate Russian bra chart that diverges from the European one. The same band ladder in centimeters, the same cup intervals, the same letters: 75B in Russia is 75B in Europe. That is a finding, not a hole in the table: in clothing the RU and EU scales differ by exactly six numbers, and out of habit people look for the same offset in lingerie, where it isn't there.

The Japanese notation is European as well, with the same band in centimeters and the same letters. Only two of the seven part company with the European row: the British and the American.

A British F and an American F are different cups

After D the British row runs on doubled letters — DD, E, F, FF — while the American one goes DD, DDD and then straight to G. That is exactly where the two rows split, and the same bust picks up different letters in each: a British F sits one step below an American G, and an American F does not exist in that row at all.

And one more thing charts usually keep quiet about: THREE American letter rows are in use. Besides the one shown here you will run into D-DD-DDD-DDDD-E-EE and D1-D2-D3, on the same bodies. The most common one is shown, but there is no standard behind those letters — which is why the basis under the American column is labeled a retail convention and not a standard.

What this arithmetic doesn't know

A difference of two measurements is one number standing in for a volume. The Wikipedia survey of practice states outright that band and cup sizes are not standardized and differ noticeably from one maker to the next, that a study found the printed size consistently unlike the measured one, and it carries a blunter verdict as well: the system is wrong often enough to be useless.

That is not a reason to withhold the number — every store asks for it. It is a reason to put the sister sizes next to it and not pass one row off as a verdict. It is also why the words «your size» do not appear on this page: the chart has an answer, the body doesn't.

Separately, about what we don't collect. The practice these intervals come from takes the bust measurement TWICE — standing, and leaning forward — and averages the two when they differ by more than 10 cm. That second reading is what rescues the method for everyone whose tissue doesn't sit where the tape finds it on a standing body. We don't ask for it, and on a single reading our answer for those bodies will come out low.

How to get your own size

To enter the two measurements and see the size in all seven notations at once, together with the sister sizes and a flag if your underbust landed close to a band boundary — open the app. You don't need an account for that. How to take both measurements, and the other twenty-six — in the measurement guide; clothing charts by bust, waist and hips — on the sizes page.