Sister Sizes Calculator
Sister sizes hold roughly the same cup volume on a different band. Go one band size up and one cup letter down, or one band down and one cup up: 34C, 32D and 36B are sister sizes. Use them when the cups fit but the band feels too tight or too loose.
Find your sister sizes
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Converted size
Brands do not all follow the same chart. Treat converted sizes as a starting point and check the brand’s own size guide.
How to use it
- Choose the size system of the size you know.
- Type your size, for example 34C, 75B or 12DD.
- Read the sister sizes table: one or two steps up and down.
- Try the sister size that fixes how your band feels.
How sister sizes work
A cup letter is relative to the band. When the band goes up by one size (2 inches), the same cup volume is one letter lower. So the cups of a 34C, a 32D and a 36B are close in volume, while the bands are different lengths.
| Band | A | B | C | D | DD | E | F | FF | G | GG |
|---|---|---|---|---|---|---|---|---|---|---|
| 28 | 28A | 28B | 28C | 28D | 28DD | 28E | 28F | 28FF | 28G | 28GG |
| 30 | 30A | 30B | 30C | 30D | 30DD | 30E | 30F | 30FF | 30G | 30GG |
| 32 | 32A | 32B | 32C | 32D | 32DD | 32E | 32F | 32FF | 32G | 32GG |
| 34 | 34A | 34B | 34C | 34D | 34DD | 34E | 34F | 34FF | 34G | 34GG |
| 36 | 36A | 36B | 36C | 36D | 36DD | 36E | 36F | 36FF | 36G | 36GG |
| 38 | 38A | 38B | 38C | 38D | 38DD | 38E | 38F | 38FF | 38G | 38GG |
| 40 | 40A | 40B | 40C | 40D | 40DD | 40E | 40F | 40FF | 40G | 40GG |
| 42 | 42A | 42B | 42C | 42D | 42DD | 42E | 42F | 42FF | 42G | 42GG |
| 44 | 44A | 44B | 44C | 44D | 44DD | 44E | 44F | 44FF | 44G | 44GG |
Moving one row down (band up 2) and one column left (cup down 1) keeps roughly the same cup volume. US letters match UK up to D.
When to use a sister size
- Band too loose, cups fit: go to the sister size down: band down one, cup up one (34C to 32D).
- Band too tight, cups fit: go to the sister size up: band up one, cup down one (34C to 36B).
- Your size is not stocked: a sister size can be a stand-in, but the band will feel different.
Limits of sister sizing
Sister sizes only keep the cup volume. Moving more than one or two steps changes the width of the cups and the position of the underwire, so the shape will not fit the same way. Brands also scale their cups differently. Sizes are not standardised between brands, countries or even styles from the same brand. A calculator gives you a good place to start; how the bra actually feels on your body decides.
Frequently asked questions
What are sister sizes?
Sister sizes are bra sizes with about the same cup volume on a different band length. To find one, move the band up one size and the cup down one letter, or the other way round. 34C, 32D and 36B are one family.
Is 34C the same as 32D?
They hold about the same cup volume, so they are sister sizes, but they are not identical bras. The 32D has a band about two inches shorter, so it feels firmer around the ribcage. Choose by how the band feels.
What is the sister size of 36DD?
The sister sizes of 36DD in UK sizing are 34E (band down, cup up) and 38D (band up, cup down). In US sizing, 36DD/E has sisters 34DDD/F and 38D. Use the calculator above for any size.
Should I go up a band or down a cup?
If only the band is too tight and the cups fit, go up one band and down one cup to keep the cup volume. Going up a band without changing the cup makes the cups larger as well.
How many sister sizes can I go?
One step in either direction usually feels similar. Two steps can still work in stretchy styles. Further away, the cup shape and wire width change too much, so the fit is rarely good.
Do sister sizes work across brands?
The rule is the same, but each brand has its own band tension and cup shapes. A sister size in one brand may fit like your usual size in another, so trying on still matters.
Last updated: October 15, 2026