What Are Sister Sizes?

Why the cup letter changes when the volume does not

Most people read a bra size as two independent facts: a band number and a cup size. In reality the cup letter has no meaning on its own. It records how many inches bigger your bust is than your band.

A D cup means a four-inch difference. On a 30 band that describes a 34-inch bust. On a 38 band it describes a 42-inch bust. Both are labelled D, and the second cup is far bigger than the first.

Now run that the other way. Suppose the cup volume you need stays fixed and only the band changes. A bigger band already makes every cup letter bigger, so you need a smaller letter to get back to the same volume. A smaller band shrinks every letter, so you need a larger one. That is all sister sizing is:

• Band down one size (two inches), cup up one letter.
• Band up one size, cup down one letter.

Sister sizes in UK lettering

The rule uses the UK cup order, which includes double letters: AA, A, B, C, D, DD, E, F, FF, G, GG, H, HH, J. "Up one letter" means the next step on that ladder, so the cup after D is DD and the cup after F is FF.

American charts skip the double letters, which is why a UK 36DD pairs with a UK 34E where a US chart shows 34DDD. For any other size, the sister size calculator will look it up.

What sister sizes are for

They solve one specific problem: the cups are right and the band is wrong.

Say you try a 34D. The cups fit well, but the band slides up your back. A 32D would give you a firmer band, but its cups are smaller, so now you would spill out. The answer is 32DD: the firmer band, with the cup volume you already know works.

They are also a reasonable fallback when a shop has sold out of your size, and a way of exploring when a new brand runs a little tight or loose in the band.

The one-step rule

Sister sizes match in volume. They do not match in every other respect. As the band grows, manufacturers scale the whole bra: the underwires get wider, the cups get broader and shallower relative to their volume, and the straps are set further apart.

One step is a small enough change that most people cannot tell. By two steps, from 34C to 38A or to 30DD, you are wearing a bra proportioned for a different frame. The wires may sit too wide or too narrow and the straps in the wrong place, even though the cup holds the right amount.

So use a sister size as a one-step adjustment. If you need to move further than that, you have most likely not found your real size yet, and it is better to measure again.

A common mistake: going up a band to fix a cup

When a bra feels tight, the instinct is to go up a band: from 34C to 36C. That changes two things at once, because a 36C has a longer band and a bigger cup. If the real problem was a cup that was too small, the bra now feels more comfortable but the band is too loose to support you.

If the cups were too small, keep the band and go up a cup (34D). If the band was too tight and the cups were fine, use the sister size (36B). Deciding which problem you have is the subject of how a bra should fit.

Frequently asked questions

Is a 32D the same as a 34C?

The cups hold the same volume, so in that sense yes. The 32D has a shorter band and slightly narrower proportions. If a 34C fits your cups but feels loose around the ribs, 32D is the size to try.

Are sister sizes a real thing or a sales trick?

They follow directly from how bras are sized. A cup letter measures the difference between bust and band, so the same difference on a bigger band is a physically larger cup. Sister sizes are the sizes where those effects cancel out.

How many sister sizes away can I go?

One step in either direction is reliable. Beyond that the wires, straps and cup shape are scaled for a noticeably different frame, and the fit usually suffers even though the volume matches on paper.