Guides / Bras / Conversions
Bra sister sizes: the same cup on a different band
Updated . 4 minute read.
The short answer
A sister size is one band down and one cup up, or one band up and one cup down. 34C, 32D and 36B all hold the same cup volume.
Use one when the cup fits and the band does not: a loose band means go down a band and up a cup.
Sister size calculator
- Two bands down, cup up30DD UK 30DD
- One band down, cup up32D UK 32D
- Your size34C UK 34C
- One band up, cup down36B UK 36B
- Two bands up, cup down38A UK 38A
A sister size holds the same cup volume on a different band. It fixes a band that is too loose or too tight without changing the cup's fit.
Why a cup letter is not a size on its own
A cup letter measures the difference between the bust and the band, so it only means something next to a band. A D cup on a 32 band holds less than a D cup on a 36 band. Move the band down two inches and the same cup needs the next letter up to hold the same volume. That is the whole rule.
When to go down a band, and when to go up
- Band rides up at the back, or you need the tightest hook on a new bra: the band is too loose. Go one band down and one cup up (34C to 32D).
- Band digs in or leaves marks, but the cup is right: go one band up and one cup down (34C to 36B).
- Cup gaps or spills: that is a cup problem, not a band problem. Change the cup letter and keep the band.
Not sure of your starting size? The bra size calculator works from a tape measure, and the bra converter goes from a size that fits at one brand to another.
Full sister size chart, US letters
| Smallest band | Sister sizes |
|---|---|
| 28B | 30A · 32AA |
| 28C | 30B · 32A · 34AA |
| 28D | 30C · 32B · 34A · 36AA |
| 28DD | 30D · 32C · 34B · 36A · 38AA |
| 28DDD | 30DD · 32D · 34C · 36B · 38A · 40AA |
| 28G | 30DDD · 32DD · 34D · 36C · 38B · 40A · 42AA |
| 28H | 30G · 32DDD · 34DD · 36D · 38C · 40B · 42A · 44AA |
| 28I | 30H · 32G · 34DDD · 36DD · 38D · 40C · 42B · 44A |
| 28J | 30I · 32H · 34G · 36DDD · 38DD · 40D · 42C · 44B |
| 28K | 30J · 32I · 34H · 36G · 38DDD · 40DD · 42D · 44C |
| 28L | 30K · 32J · 34I · 36H · 38G · 40DDD · 42DD · 44D |
| 28M | 30L · 32K · 34J · 36I · 38H · 40G · 42DDD · 44DD |
| 28N | 30M · 32L · 34K · 36J · 38I · 40H · 42G · 44DDD |
| 28O | 30N · 32M · 34L · 36K · 38J · 40I · 42H · 44G |
| 30O | 32N · 34M · 36L · 38K · 40J · 42I · 44H |
| 32O | 34N · 36M · 38L · 40K · 42J · 44I |
| 34O | 36N · 38M · 40L · 42K · 44J |
| 36O | 38N · 40M · 42L · 44K |
| 38O | 40N · 42M · 44L |
| 40O | 42N · 44M |
Full sister size chart, UK letters
| Smallest band | Sister sizes |
|---|---|
| 28B | 30A · 32AA |
| 28C | 30B · 32A · 34AA |
| 28D | 30C · 32B · 34A · 36AA |
| 28DD | 30D · 32C · 34B · 36A · 38AA |
| 28E | 30DD · 32D · 34C · 36B · 38A · 40AA |
| 28F | 30E · 32DD · 34D · 36C · 38B · 40A · 42AA |
| 28FF | 30F · 32E · 34DD · 36D · 38C · 40B · 42A · 44AA |
| 28G | 30FF · 32F · 34E · 36DD · 38D · 40C · 42B · 44A |
| 28GG | 30G · 32FF · 34F · 36E · 38DD · 40D · 42C · 44B |
| 28H | 30GG · 32G · 34FF · 36F · 38E · 40DD · 42D · 44C |
| 28HH | 30H · 32GG · 34G · 36FF · 38F · 40E · 42DD · 44D |
| 28J | 30HH · 32H · 34GG · 36G · 38FF · 40F · 42E · 44DD |
| 28JJ | 30J · 32HH · 34H · 36GG · 38G · 40FF · 42F · 44E |
| 28K | 30JJ · 32J · 34HH · 36H · 38GG · 40G · 42FF · 44F |
| 30K | 32JJ · 34J · 36HH · 38H · 40GG · 42G · 44FF |
| 32K | 34JJ · 36J · 38HH · 40H · 42GG · 44G |
| 34K | 36JJ · 38J · 40HH · 42H · 44GG |
| 36K | 38JJ · 40J · 42HH · 44H |
| 38K | 40JJ · 42J · 44HH |
| 40K | 42JJ · 44J |
Where the UK and US letters part, and what each brand prints, is in UK vs US bra sizes.
Common questions
What is a bra sister size?
A size with the same cup volume on a different band. Going one band size down and one cup up (34C to 32D), or one band up and one cup down (34C to 36B), keeps the cup the same size.
When should I wear a sister size?
When the cup fits but the band does not. A band that rides up or needs the tightest hook when new is too loose: go one band down and one cup up. A band that digs in: go one band up and one cup down.
Are sister sizes the same in UK and US sizing?
The rule is the same, but the letters differ after DD. The UK goes DD, E, F, FF; the US goes DD, DDD, G, H. The calculator on this page prints both.
How far can I go with sister sizes?
One step is the usual fix. Two steps away the band and the underwire width change enough that the bra fits differently, even though the cup volume matches.
Sources
Checked on 19 September 2026. Brand charts change: the linked page is the authority, not this one.
- Wikipedia: Bra sizeUsed for: band and cup systems, and why cup letters are relative to the band.
Skip the conversion table
Tell us one item you own and how it fits. TrueToSize reads the brand's own chart and gives you your size at other brands, with a confidence level on every answer.