B cup vs C cup

One inch. That is the whole difference - a B cup is two inches larger around the bust than around the ribcage, and a C cup is three. Everything else on this page follows from that.

34B

2" diff

1.48" deep

6.13" across

34C

3" diff

1.84" deep

6.37" across

Looking down from above - 34B and 34C

34B

34C

Sliced across the body at the fullest point and seen from above. The dashed line is the ribcage on its own - the 34 band. A bigger cup does not only stand further out from the chest, it covers more of it, and the gap at the breastbone narrows. That is the part a side view cannot show you.

A bigger cup does not mean a lower one. The shapes barely change with size because size is not what decides how far a breast sits down - weight and having breastfed matter far more.

Side profiles on a matched 34 band, drawn at the projection the model solves for each cup. The vertical line is the chest wall. One inch of extra difference buys 0.36 of an inch of extra projection - which is why the two look so similar here and behave so differently in a garment.

What the inch is actually worth

Circumference is a poor guide to volume, because an inch added to a circle's perimeter buys much less than an inch of depth. Running both through a volume model on a 34 band:

34B - about 405 mL per breast, roughly 385 g.
34C - about 545 mL per breast, roughly 518 g.

That is about 35% more tissue, and around 266 g more for the pair - about the weight of a small apple. Visually it is a few millimetres of projection. In a bra it is the difference between a cup that encloses the breast and one that compresses the top of it, because cup depth is not a gradual thing: either the tissue fits inside the cup or it does not.

A B cup is not one size

The most common mistake in this comparison is treating the letter as an absolute. It is not. Two inches of difference around a 30-inch ribcage and two inches around a 40-inch ribcage produce noticeably different amounts of tissue, because the same surplus is being wrapped around a much larger frame.

Modelled volume per breast for B and C cups across bands.
Band	B cup	C cup	The step between
30	125 mL	265 mL	+140 mL
32	265 mL	405 mL	+140 mL
34	405 mL	545 mL	+140 mL
36	545 mL	685 mL	+140 mL
38	685 mL	825 mL	+140 mL
40	815 mL	965 mL	+150 mL

A 40B holds more than a 30C. The letter on its own carries almost no information about size without the band beside it.

34B and 32C are the same bra size

They are sister sizes. Tighten the band from 34 to 32 and the difference between bust and band grows by two inches - one of which is absorbed by moving from a B to a C. The volumes come out at 405 mL and 405 mL. They are not identical - the model reads the wider band a little lower, as it does all the way along the sister ladder - but they are far closer to each other than either is to a 34C.

So "am I a B or a C" is often the wrong question. If your band is right and the cup is marginal, the letter is the answer. If the band creeps up your back, the honest answer is that you are a smaller band and a larger letter than either - try the sister sizes.

Telling them apart on yourself

The boundary is one inch wide, which is inside the error of a badly taken measurement. Three things push a real C into reading as a B:

A padded bra during measuring pushes the other way, inflating the bust reading by an inch or two and turning a B into a C or a D on paper.
A tape riding up at the back adds to the underbust more than the bust, shrinking the difference and reading a C as a B.
Measuring at different points in a cycle. Bust circumference can move by a full inch across a month, which is the entire gap between these two letters.

If your difference lands within half an inch of three inches, you are on the boundary and both are worth trying. That is not a failure of the calculator; it is a one-inch grid applied to a body that does not come in one-inch steps.

In other countries

B and C are the two letters where every system in the world agrees, which is unusual - the divergence only begins above D. A 34C is a 34C in the UK, a 75C in Europe, a 12C in Australia and a 90C in France. Only the band notation changes.

Bra size calculator - find out which of the two you are.
DD cup size - where the systems stop agreeing.
Sister size calculator.
Breast weight calculator.
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Questions people actually ask

What is the difference between a B cup and a C cup?

Exactly one inch of difference between the bust and the band. A B cup is two inches larger around the bust than around the ribcage; a C cup is three. On a 34 band that is the difference between roughly 405 mL and 545 mL of tissue per breast - about 35% more tissue - a bigger jump than the single letter suggests.

Is a C cup noticeably bigger than a B cup?

On the body, less than most people expect; in a garment, more than most people expect. One inch of extra difference translates to only a few millimetres of extra projection, so the visual difference is modest. But cup depth is where the bra either contains the tissue or cuts across it, and that boundary is sharp - which is why a B that spills and a C that fits look almost identical on paper and completely different when worn.

Is a 34B bigger than a 32C?

No - if anything the 32C is fractionally the larger of the two. They are sister sizes: a 34B is two inches of difference on a 34 band, a 32C is three inches on a 32 band, and the extra cup letter is meant to offset the two inches lost from the band. Modelled, they come out at 405 mL and 405 mL per breast, so the offset is close rather than exact. What really differs is the fit: the 32C anchors on a tighter band and the 34B leans more on the straps.

Is a B cup or C cup more common?

Retail stock is heavily weighted toward B and C on 34 and 36 bands, but that reflects the +4 fitting method rather than what people measure. When band and cup are measured directly, the distribution shifts markedly toward smaller bands and larger letters - the same bodies simply get relabelled.

Does a B cup mean the same thing on every band?

No, and this is the part that matters most. A B is always two inches of difference, but two inches measured around a 30-inch ribcage is a smaller breast than two inches around a 40-inch one. This model puts a 30B at about 125 mL per breast and a 40C at about 965 mL. Comparing bare cup letters between people tells you very little.

How do I know whether I am a B or a C?

Measure rather than guess, because the boundary is one inch wide and a tape that has ridden up at the back will cross it on its own. Take the underbust snug and the bust loose, subtract, and if the answer lands within about half an inch of the boundary, try both - the sister sizes on either side are worth more than the letter.