Bra sister sizes: chart and fit guide
Learn how band and cup labels change together, and when a nearby bra size may be worth trying.
What are bra sister sizes?
Bra sister sizes pair a different band with a compensating cup step to describe approximately similar cup volume. They are alternatives to try, not identical garments or guaranteed fits.
Within the same sizing system, go down one band step and up one cup step for a tighter-band alternative. Go up one band step and down one cup step for a looser-band alternative. In US and UK labels, one band step is usually two numbers.
Bra sister size chart: US label examples
Read each row across. For example, US 34D has nearby sister labels 32DD and 36C. These examples follow this calculator’s general US label sequence; a brand may use different labels or grading.
| Tighter band | Starting size | Looser band |
|---|---|---|
| 30DD | 32D | 34C |
| 32DD | 34D | 36C |
| 34DD | 36D | 38C |
| 36DD | 38D | 40C |
When should you try a sister size?
If the cups feel comfortable but the band rides up or feels loose on the loosest hooks, try one smaller band and one larger cup step. If the cups feel comfortable but the band is uncomfortably tight, try one larger band and one smaller cup step.
If the band feels secure but the cups spill or gape, do not change the band just to follow a sister-size chart. Try another cup size in the same band, or a different bra shape. A cup that is too small can also make the whole bra feel tight.
Check that the band stays level, the cups contain your tissue without cutting in, and the straps do not carry all the support. Underwires should not sit on breast tissue. For a wired style, the centre panel often rests against the chest, but construction and body shape affect this.
Are sister sizes exactly the same?
No. Changing the band changes tension, strap spacing, wire shape and proportions. Similar cup volume does not mean equal support or comfort. Staying one band step away is a more useful first comparison than making large jumps.
A cup letter has no fixed volume without its band: 32D and 38D are not the same cup volume. Do not mix US and UK cup steps without checking which system and brand you are using.
Find a starting size before comparing alternatives
Use the bra size calculator with your current underbust and bust measurements, then consider nearby labels based on how a bra actually fits. Use the measurement guide if you are unsure where to place the tape.
Manufacturer body-measurement results and general label conversions serve different purposes. Check the brand’s official size chart and the style’s available sizes before ordering.