Bra Sister Sizing Guide & Reference Chart
Learn how bra cup volumes correspond across different band sizes. If a band feels slightly too snug or loose, use sister sizing to adjust the fit.
What is a bra sister size?
A sister size is a bra size that holds the exact same cup volume as your starting size, but utilizes a different band length. In bra tailoring, as band length increases, the physical cup size of the same letter also increases. For instance, a 36D cup is physically larger than a 32D cup.
The Golden Rule of Sister Sizing
If you want to change bands while preserving your current cup volume, apply these rules:
- Go down 1 band size → Go up 1 cup letter (e.g. 34D sister sizes down to a tighter 32DD).
- Go up 1 band size → Go down 1 cup letter (e.g. 34D sister sizes up to a looser 36C).
D Cup Sister Sizing Reference Chart
Find your starting size to see its corresponding tighter and looser sister sizes.
| Tighter Band (Go Up 1 Cup) | Core D Cup Size | Looser Band (Go Down 1 Cup) |
|---|---|---|
| 28DD | 30D | 32C |
| 30DD | 32D | 34C |
| 32DD | 34D | 36C |
| 34DD | 36D | 38C |
| 36DD | 38D | 40C |
| 38DD | 40D | 42C |
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Read reviews from women who found their perfect supportive D-cup fits.
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D Cup Bra Sizing Guidance FAQ
Answers to the most common questions about D-cup fitting and support.
What are sister sizes in D cup bras?
Sister sizes are alternative bra sizes that hold the exact same cup volume. They allow you to change the band length without changing the cup volume. Examples include 32DD and 36C for a 34D bra.
What is the golden rule of sister sizing?
The golden rule states that going down 1 band size requires going up 1 cup letter. Conversely, going up 1 band size requires going down 1 cup letter. This preserves the physical cup volume.
Why would I need to wear a sister size?
You need a sister size when the cup fits correctly but the underband feels uncomfortable. A looser band provides breathing room, whereas a tighter band offers firmer support.