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Bra Sister Size Calculator

Sister sizes hold the same cup volume on a different band. Enter the size you wear to see the sizes one and two bands up or down, and where they sit on a full band-by-cup grid.

Last updated:

Band number + cup letter, e.g. 36DD (US) or 80D (EU).

Enter your values to see the result.
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How to use the Bra Sister Size Calculator

  1. Pick your sizing region, usually the country printed first on your bra label.
  2. Type the size you wear, for example 34C, 32DD or 80D.
  3. Press Find sister sizes to see band-down/cup-up and band-up/cup-down options.
  4. Look at the grid: the dark cell is your size and the green diagonal holds your sister sizes.
  5. Choose the sister that fixes your fit problem: band too loose means go down a band, band too tight means go up.

Why sister sizes work

A cup letter is not a fixed volume. It describes how much larger the bust is than the band, so the actual cup gets bigger as the band grows: a 38C cup holds more than a 32C cup. Moving one band up adds about the same volume as one cup letter, which is why you drop a letter to stay at the same volume.

In the grid above this means every diagonal from upper right to lower left is one cup volume. Your sister sizes are the cells on your diagonal; the cells on the neighboring diagonals are one cup volume larger or smaller.

Sister size chart for common sizes (US)

Band down 2Band down 1Your sizeBand up 1Band up 2
28C30B32A34AA—
28DD30D32C34B36A
30DD32D34C36B38A
30DDD32DD34D36C38B
32DDD34DD36D38C40B
34G36DDD38DD40D42C
36G38DDD40DD42D44C

Which way should you go?

  • Band rides up your back or you are on the tightest hook: the band is too loose. Go down a band and up a cup (36B → 34C).
  • Band leaves marks or feels hard to breathe in: go up a band and down a cup (34C → 36B).
  • Cups gape or overflow: a sister size will not fix it, because it keeps the same volume. Change only the cup letter instead.
  • Most support comes from the band, so when in doubt, choose the firmer (smaller) band and the matching larger cup.

Need the same sister size in another country's sizing? The Bra Size Converter translates any size between US, UK, EU, French, Australian and Japanese labels.

Frequently asked questions

What is a bra sister size?

A sister size is a different band and cup combination with about the same cup volume. Cup volume depends on the band, so 32D, 34C and 36B all hold roughly the same amount even though the letters differ.

How do I find my sister size?

Go up one band size and down one cup letter (34C to 36B), or down one band size and up one cup letter (34C to 32D). You can repeat the step, but the fit changes more the further you move from your usual size.

When should I wear a sister size?

When the cups fit but the band does not. If the band rides up or feels loose, go down a band and up a cup. If the band digs in, go up a band and down a cup. If the cups themselves are wrong, change the cup letter instead.

What is the sister size of 34C?

The closest sister sizes of 34C are 32D (smaller band) and 36B (larger band). One step further are 30DD and 38A.

Do sister sizes work the same in UK sizing?

Yes, as long as you follow the UK cup sequence, which includes doubled letters: AA, A, B, C, D, DD, E, F, FF, G, GG. So the sister size of UK 32FF one band up is 34F, not 34E.

Is a sister size exactly the same fit?

No. The cup volume is similar, but the wire width, strap position and band firmness change with the band. Sister sizes one step away usually work well; two or more steps away often feel noticeably different.

Are all the tools on FreeToolCabin tested for quality and accuracy?

Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.

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