What Does 34B Mean? Bra Band Numbers and Cup Letters

A bra size of 34B means a 34-inch band and a B cup. The 34 is the band, sized from a snug measurement around your ribcage just under the bust. The B is not a volume: it means your bust measures about 2 inches more than your band. Because the cup letter is a difference, a B cup on a 38 band holds more than a B cup on a 34.

The 34: a band measured around the ribcage

Wrap a tape measure snugly around your torso at the fold under the breasts, level all the way round, and read it in inches. Most modern fit guides round that number to the nearest even figure and use it as the band, which is why bands come in 30, 32, 34, 36 and so on. A ribcage of 33.5 inches lands at a 34.

The plus-four method, and why some 34s are really 30s

Older charts added inches to the ribcage figure: 4 if the measurement was even, 5 if it was odd. Wikipedia's bra-size entry notes that Kohl's still used this "underbust plus four" method in 2013, and that a campaign known as the "war on plus four" was arguing against it around 2011. The Lingerie Addict, writing about vintage bras, explains that women once added 4 inches to the band and subtracted from the bust to get the cup, and that vintage sizes therefore "skew towards larger bands and smaller cups."

The method changes the answer a lot. Take a ribcage of 30 inches and a bust of 36:

Same body, two labels four cup sizes apart. If a store calculator told you 34B, check which method it used before you trust it.

The B: a cup letter is a difference

Measure loosely around the fullest part of the bust, tape level, then subtract the band number. In the US and UK each inch of difference moves you one cup letter:

So a 34B is a woman with roughly a 34-inch ribcage and a 36-inch bust. Wikipedia puts the step at "always one inch, or 2.54 cm" for US and UK sizing. Continental Europe measures bands in centimeters and steps cups every 2 cm, which is why a US or UK 34B is sold as 75B in Germany or Italy and 90B in France, and why the letters drift apart further up the alphabet. Up to D, the letters line up in most systems; past DD they do not.

Why a 34B and a 38B are different cups

A B cup on a 38 band has to wrap around a wider chest, so the wire is longer and the cup deeper. Wikipedia's entry reproduces underwire charts from the pattern-maker Don McCunn that make this concrete. In that table, a 34B uses the same wire as 36A, 32C and 30D, a cup of about 390 cubic centimeters. A 38B shares its wire with 40A, 36C and 34D, at about 590. That is half again as much volume behind the same letter.

Read the other way, the cup of a 38B is roughly the cup of a 34D. That is why the letter on its own says so little about breast size.

Sister sizes

The sizes that share a wire are called sister sizes. For 34B they are 36A and 32C: same cup volume, different band. Move one band size up and one cup letter down (or the reverse) and the cup stays about the same while the band changes. It helps when a brand's band runs tight or loose. It is an approximation, and it does not fix a cup that is the wrong shape for you.

Where the letters came from

Lettered cups are a 1930s invention, and the record is untidy. Wikipedia credits S.H. Camp and Company with the first A to D letters in October 1932, though its letters described how pendulous the breasts were rather than their volume. The same year, according to the book Uplift: The Bra in America as quoted by the Underpinnings Museum, Formfit announced three cup volumes in each band size, labeled small, average and full. Warner's is usually credited with the Alphabet Bra of 1935, sized A to D by volume, though Uplift says Warner did not feature cup sizing until 1937. In Britain, the Underpinnings Museum notes, many makers still labeled cups "medium" or "average" into the late 1950s.

What the 1930s did not produce was a standard. Wikipedia notes that band sizes vary between manufacturers and that US brands disagree past D, some running DD, DDD, DDDD, others jumping to G. A 34B from one label and a 34B from another can fit differently, and an underwire bra with a narrow wire will fit differently from a wireless bra in the same size.

So is 34B small?

On a 34 band it is a modest cup, about the same as a 32C. It is not small in any absolute sense. Wikipedia cites a 2012 study of 45 women in which 76% overestimated their band and 84% underestimated their cup, the classic error being a band too big and a cup too small. A woman wearing 34B because a plus-four calculator said so may well be a 30 or 32 band with a C, D or larger cup, which is why a tape measure is a starting point and the fit in the fitting room has the last word. Our bra type guide covers which cuts suit a smaller or fuller cup once the size is settled.

Last updated November 7, 2026

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