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Sister Size Calculator
Sister sizes hold the same cup volume on a different band. Enter your size to see the band-up and band-down alternatives — useful when the band feels off or your size is out of stock.
Measure the underbust snugly around your ribcage, and the bust around the fullest part, both level to the floor.
Enter your measurements above to get your bra size in every standard.
How sister sizing works
Cup volume depends on the band as well as the letter. To keep the cups the same size while changing the band, you move the cup letter the opposite way:
- Band too loose? Go down a band and up a cup — e.g. 34C → 32D.
- Band too tight? Go up a band and down a cup — e.g. 34C → 36B.
The cups stay identical; only the band and the letter on the label change.
Related
Frequently asked questions
What is a sister size?
A sister size is a different bra size with the same cup volume. When you change the band by one step, you change the cup by one letter in the opposite direction to keep the cups the same size. For example, 34C, 32D, and 36B all hold the same cup volume.
When should I try a sister size?
Try going down a band (and up a cup) if the band rides up your back or feels loose; try going up a band (and down a cup) if the band digs in or the cups feel tight overall. Sister sizes are also handy when your exact size is sold out.
Does the cup letter really change for the same cups?
Yes — cup volume depends on both the letter and the band. A C cup on a 34 band is larger in volume than a C cup on a 30 band. That's why the letter shifts when the band does, even though the actual cups are identical.
How do I find my sister size?
Enter your current size in the converter above. Its sister sizes — one band down and one band up — appear automatically, along with the international equivalents.