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.
Processed entirely in your browser — nothing you enter here is ever uploaded.
Result
About this tool
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.
Example. An underbust of 30 in and a bust of 35 in produce the US size 34A, with sister sizes 32B and 36AA.
Limit. The cup index is clamped from AA through L, a bust that is not larger than the underbust is rejected, and convert mode accepts band numbers only from 20 to 60.