Bra Sizes Explained: What the Letters and Numbers Mean

The number on a bra tag is the band, and the band is the measurement around your ribcage directly under the bust, taken snugly, with the tape level and exhaled. In the US, UK and Australia it is written in inches, rounded to the nearest even number, so a 31-inch underbust becomes a 32 band and a 33 becomes a 34. In continental Europe the band is written in centimetres, in France and Spain as the European number plus 15, in Australia as a dress size, in Italy as a small index. These convert from the label, not from your tape: a US or UK 32 is sold as an EU 70, a 34 as a 75, a 36 as an 80, and that is the number to look for whatever your ribcage measures in centimetres.

That last point trips up careful people, so it is worth being explicit. Measure a 34-inch ribcage in centimetres and you get 86, yet the bra that fits it is labelled 75 in Europe. The EU scale was calibrated in the era of the add-four rule, when a 30-inch ribcage was sold a 34 band, and the labels have stayed aligned to each other rather than to the tape ever since. So the rule is: measure once in whatever unit you like, find your US or UK band by the snug method, and convert the label. Never re-measure in centimetres to find a European size; you will land two bands too large.

What the band is not is the number you get by adding four or five inches to your underbust. That method dates from an era of rigid, non-stretch fabrics and it is still printed on many retailers’ charts; it produces bands two sizes too loose for modern elastic, and a loose band is the root of most fit complaints on this site. The band does roughly ninety percent of the supporting work, so the number matters more than the letter. Measure it snugly and trust it.

Take a D cup, four inches of projection, and put it on three ribcages. On a 32 band the four inches sit on a narrow base and the cup holds a modest volume. On a 36 the base is wider and the same four inches of projection enclose a good deal more. On a 40 the cup is wider again and the volume larger still. Clinical measurements of bra cups and excised tissue put the difference at roughly two and a half times between a 32D and a 40D, for the same letter. The letter told you the projection; the band told you the width; volume is both.

The table below puts numbers on it, using the estimates the breast-biomechanics literature uses (grams of tissue per breast, and breast tissue is close enough to the density of water that grams and millilitres are interchangeable). Read across a row and the same letter roughly doubles; read down a column and each letter adds a step whose size depends on the band. The breast weight calculator explains where the figures come from and how wide their range is.

Central estimates per breast; dashes where the linear model falls below 100 g. Bands 30 and 40 are cross-graded from the studied 32 to 38 range.

If the letter is a difference and the number is a base, then a bigger base with a smaller difference can enclose the same volume as a smaller base with a bigger one. That is a sister size. Go down a band and up a letter, or up a band and down a letter, and the cup holds the same amount: a 32DD, a 34D and a 36C are one cup volume with three labels. The band changes; the breast that fits it does not.

Sister sizing is the most useful idea in bra fitting after measuring correctly, because it separates the two things a bra can get wrong. Cups fit but the band rides up? Move down a band and up a letter, keeping the volume. Band fits but the cups gape? Move down a letter and leave the band alone; that is not a sister move, it is a volume change. Fitters use the first constantly; shoppers confuse it with the second constantly. The sister size calculator returns both sisters for any size and the table below shows the pattern.

The ladder is one thing; tags are another. Several habits on real labels trip people who have just learned the system, and every one of them resolves by reading position instead of letter.

Dual labels. Some US-market brands print the contested cups both ways, E/DD and F/DDD, so a US customer and a European one land on the same row. Montelle does this on its published chart. Read either label as the fifth and sixth positions; the slash is a translation, not a size between two sizes.

The retail DDD/F convention. Many US retailers treat DDD and F as the same six-inch cup, then continue G, H. That is a US ladder with F standing in for DDD. It is fine until a UK-lettered bra arrives on the same rail, where F is the seventh cup; a store that mixes the two is selling two different sizes under one letter, and the tag’s country of origin is the only way to know which.

Japanese cups. Japanese bands use the EU centimetre scale and Japanese cups follow the EU alphabet by position, but some Japanese brands cut each letter one step deeper than the European equivalent for the same measurement. Treat the JP label as the one to look for, then check the brand’s own chart.

Alpha over structured. A tag reading M or L is a box, not a size. If the brand also publishes a structured chart, that chart is the truth and the letter is the compression of it.

The number is not your centimetres. As explained above, an EU 75 is the bra a US 34 wears, not a 75-centimetre ribcage. Convert the label; never re-measure into a European size.