Bras · Sizing
Bra sister size calculator.
Enter your band and cup in US or UK lettering. You get the up-band and down-band sister sizes that hold about the same cup volume.
Your bra size
Sister sizes
Enter your band size and cup to see your sister sizes.
How sister sizes are worked out
The same-volume rule. A band is the underbust measurement and a cup is the difference between bust and band. When the band gets 2 inches bigger, the cup letter that holds the same volume is one letter smaller, because the cup sits on a larger, longer band. When the band gets 2 inches smaller, the cup must be one letter bigger to hold the same volume.
- Up-band sister: band +2, cup one letter smaller (34DD to 36D).
- Down-band sister: band -2, cup one letter larger (34DD to 32DDD).
- Two steps: repeat the move (34DD to 38C or 30G). Fit gets less reliable the further you move from your own size.
Worked example. A 36C that is tight on the loosest hooks can move to a 38B (up-band) for a longer band with the same cup volume, or loosen the fit the other way with a 34D (down-band) for a snugger band.
US and UK cup letters
Letters match up to D. After that: US usually DD (E), DDD (F), G; UK DD, E, F, FF, G, GG, H, HH, J, JJ, K. Pick your lettering system and the tool steps along that ladder. It stops at the end of the supported letters (AA to J in US, AA to K in UK) and at bands 28 to 52, rather than inventing a size.
This is a convention. Brands, countries and even lines within a brand size differently, and sister sizes ignore the shape of the breast, the width of the wires and the length of the band. Treat the result as sizes to try on, and use a professional fitting if you are unsure. Nothing you enter is stored or sent.
Common sister-size examples
Use the calculator for any supported US or UK size. Common US examples include 34DD → 36D or 32DDD, 36C → 38B or 34D, and 38DDD → 40DD or 36G. One more step keeps the nominal cup volume similar, but fit becomes less reliable as the band moves farther from your measured size.
Frequently asked questions
What does sister size mean in bras?
A sister size is a different band and cup combination that holds about the same cup volume. You go up one band size (+2) and down one cup letter, or down one band size (-2) and up one cup letter. 34DD, 36D and 32DDD are sisters.
Which direction do I go if my band feels too loose or too tight?
If the band is too loose, try the down-band sister: a smaller band with a bigger cup letter, for example 36C to 34D. If the band is too tight, try the up-band sister: a bigger band with a smaller cup letter, for example 34D to 36C. The cup volume stays about the same either way.
Are sister sizes exactly the same size?
No. Only the nominal cup volume is held roughly constant. A larger band also has a wider cup, and a smaller band a narrower one, so the shape and the way the wires sit change. Brands also cut differently, so a sister size is a place to start trying on, not a guarantee.
Do US and UK cup letters match?
They match up to D. After that the systems differ: US sizing usually goes DD (also written E), DDD (F), then G, while UK sizing goes DD, E, F, FF, G, GG, H and so on. This calculator steps along the ladder of whichever system you choose and does not convert between them, because brands differ beyond F.
Why does the calculator stop at some sizes?
It will not invent a size past the end of the cup letters it supports, or a band outside 28 to 52. For example there is no up-band sister of an AA cup, because that would need a cup smaller than AA.
What is the sister size of 34DD, 36C or 38DDD?
34DD has up-band sisters 36D and 38C and down-band sisters 32DDD and 30G in US lettering. 36C has 38B and 34D. 38DDD has 40DD and 36G. Enter any of those sizes above for the full result.
Sources
Reviewed October 2, 2026.
- The band +2 / cup -1 rule is a bra-fitting convention. No standard defines sister sizes, so there is no standard to cite for the rule itself.
- Bust and underbust body measurement definitions: ISO 3635 and the EN ISO 13402 series (parts 1 and 3), cited by number only. Their tables are not reproduced here.
- Cup letters beyond D are brand and country specific. This page uses the common US ladder (DD/E, DDD/F, G) and UK ladder (DD, E, F, FF, G, GG), as a convention.
Scope
A sizing guide to help you choose what to try on. Fit varies by brand, style and body shape. It is not a professional fitting.