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Bra Sister Size Calculator

Estimate your bra size from underbust and bust measurements, or convert an existing size between US, UK, EU, FR and IT — plus its sister sizes.

Measure snugly (not tight) with a soft tape: underbust right beneath the bust, bust at its fullest point.

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Reading measurements
Matching sizes
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Result

This page opens the bra size calculator with the sister-size explainer front and centre: measure your size or convert one you already know, and the two sister sizes appear right alongside the main result.

A sister size is a different band-and-cup combination that holds roughly the same cup volume as your actual size — moving one band size up while dropping one cup letter, or one band size down while going up a cup letter. A 34B, for example, shares its cup volume with a 36A (bigger band, smaller-lettered cup) and a 32C (smaller band, bigger-lettered cup); all three describe a similarly sized cup, just distributed across a looser or snugger band. That makes sister sizes genuinely useful when a brand doesn't stock your exact size, or when a band feels a little loose or tight but the cup itself fits well.

The complete calculator lives on this page — switch between measuring and converting, or change the size you start from, and the sister sizes recalculate automatically. Every result is computed locally in your browser from static published tables, with no uploads and no account required.

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Executed in Browser

Your files and inputs never leave this device. Processing runs completely offline. 100% Secure.

Accepted Formats & Rules

  • Traditional and modern band formulas from underbust, plus bust-minus-band mapped through an AA-to-L cup table.
  • Size conversion between US, UK, EU, FR and IT band numbers and cup letters, using real published band and cup charts.
  • Sister size calculator for whichever size you measured or converted: one band up/cup down, one band down/cup up.

Key Benefits

✓ Two modes in one tool: measure your size from underbust and bust, or convert a size you already know between five sizing systems.
✓ Cup letters are matched with a published international equivalence table, not a rough guess — with an honest note on where systems disagree.
✓ Every result also shows its two sister sizes — same cup volume, different band and cup letter — so you know what else to try.

Use Cases

  • • Never measured your bra size before and want a solid starting point before you shop?
  • • Ordering lingerie from a UK or European site and only know your US size?
  • • Curious what a US 34DD actually is in EU or French sizing?
  • • A brand is sold out of your size — want to know which sister sizes to try instead?