What is a sister size?

If your bra is almost right — the cup is good but the band feels off, or the band fits but the cup is wrong — the answer is rarely a wholesale size change. It is your sister size, the small adjustment that fitters use most.

Sister sizes are one of those simple ideas that solve a surprising number of fit problems. Once you know your starting size from the calculator, your two sister sizes are the next things worth knowing.

What a sister size actually is

A sister size is a different bra size with the same cup volume. The numbers and letters change, but if you filled the cup with water, both sizes would hold the same amount.

The rule that creates sister sizes:

• Going down a band size requires going up a cup letter to keep the cup volume the same
• Going up a band size requires going down a cup letter

That is the whole concept. If your size is 34D, your sister sizes are 32DD (smaller band, bigger cup) and 36C (bigger band, smaller cup). The cups in all three sizes hold the same amount.

Why sister sizes exist at all

Cup letters do not refer to a fixed volume. A C cup on a 32 band is much smaller than a C cup on a 40 band — because the larger band has to wrap around a larger torso, and the cup is proportionally bigger too. Cup letters are always relative to their band.

This means there is more than one way to label any given cup volume. A specific amount of bust tissue can be described as a 32DD, a 34D, or a 36C — same volume, different label. Each option is a sister to the others.

When to try a sister size

The cup fit usually tells you about cup volume; the band fit usually tells you about band size. Match the symptom to the fix.

Your cup fits perfectly, but the band feels too loose

The band needs to be smaller. Drop down a band size and go up a cup letter — that is your sister size down. 34D → 32DD.

Your cup fits perfectly, but the band digs in

The band needs to be bigger. Go up a band size and down a cup letter — your sister size up. 34D → 36C.

The band feels right, but you are overflowing the cups

That is a cup volume problem, not a sister-size problem. Stay in the same band, go up a cup letter. 34D → 34DD.

The band feels right, but the cups are wrinkling

Also a cup volume problem in the other direction. 34D → 34C.

The shortcut: if the band is wrong, change to a sister size. If the cup volume is wrong, change the cup letter without touching the band.

A worked example, end to end

You measure yourself and the calculator says 34D. You buy a 34D. It feels almost right — the cup is comfortable, the gore lies flat — but the band rides up at the back when you raise your arms.

The band riding up means the band is too loose. Your cup is fine. So you stay with the same cup volume but go down a band: 34D becomes 32DD. Your sister size down.

You order a 32DD in the same brand and style. This time the band stays put. Same brand, same style, one sister size away — solved.

Your full sister-size reference

Here are the sister sizes for the most common UK calculated sizes. Find your starting size in the middle column, and the two sisters either side are the alternatives to try if the main one is not quite right.

UK sister sizes

Sister size (down a band)	Calculated size	Sister size (up a band)
30C	32B	34A
30D	32C	34B
30DD	32D	34C
30E	32DD	34D
32D	34C	36B
32DD	34D	36C
32E	34DD	36D
32F	34E	36DD
34DD	36D	38C
34E	36DD	38D
34F	36E	38DD
34FF	36F	38E

Common sister-size mistakes

Going up only in the band when something feels tight

If 34D feels tight in the band, lots of people grab 36D thinking they need a bigger band. But 36D is not 34D's sister — it has both a bigger band and a bigger cup. You have changed two variables. The actual sister is 36C — bigger band, same cup volume.

Going up only in the cup when something feels small

Same problem in the other direction. If 34D feels too small in the cup, going to 36D adds band size you may not need. Try 34DD first.

Trying sisters three steps away

30G is technically a "sister" of 34D in a mathematical sense, but at that distance the cup proportions are too different. Stick to one step in either direction. If neither works, the brand or style is not right for your shape — try another.

Assuming sister sizes fit identically

They hold the same cup volume, but the shape of that volume is slightly different. A 32DD has a deeper, narrower cup than a 36C with the same volume — the smaller band means the cup wraps around a smaller torso and pushes the volume outward instead of sideways. For most people the difference is minor, but if you are between sister sizes and unsure which to pick, try both.

One last thing

Sister sizes are a tool, not a magic fix. If your calculated size is 34D and neither 32DD nor 36C works, the answer is rarely a fourth sister — it is usually that the brand or style does not suit your bust shape. Try a different brand before going further from your starting size.

But for most fit problems? A sister size is the cheapest, simplest experiment you can run. One band hook in a different direction, one letter shift, and the same bra often becomes the right bra.