Sister Size Calculator – Find Your Equivalent Bra Size

What Are Sister Sizes?

Sister sizes are bra sizes that hold the same cup volume even though the band and cup letter are different. For example, 34C, 32D, and 36B all have the same cup capacity. If your usual size is sold out or feels a bit off, a sister size can give you the same support with a different fit.

Why Use Sister Sizes?

• Your size is out of stock – sister sizes give you more options.
• Different brands fit differently – a sister size may fit better in a specific brand.
• Your body changes – weight loss, gain, or pregnancy can change how your bra fits; sister sizing gives you flexibility.
• Comfort – if your current bra digs in or rides up, a sister size might solve the problem.

How to Use the Sister Size Calculator

Simply enter your current band number and cup letter in the tool above. The calculator will instantly show you the two sister sizes – one with a smaller band and larger cup, and one with a larger band and smaller cup. Try them on to see which feels best.

What If I’m Between Sister Sizes?

If neither sister size feels quite right, you can also try going down a band and up two cups (e.g., 34C → 30E) or up a band and down two cups (e.g., 34C → 38A). This works if you need more or less volume while keeping the same band comfort.

Why Trust This Calculator?

• Free – no sign‑up, no hidden charges.
• Private – all calculations run in your browser – we never see your measurements.
• Accurate – uses standard sister‑size logic that professional fitters use.
• Updated – our charts are checked regularly to match current brand sizing.

Your sister sizes are a great starting point. Every bra fits a little differently, so always try on a few styles to find the one that feels best.