Bra Size Calculator is a free browser-based tool that turns underbust and bust measurements into a starting band and cup size, with no account needed. It has a quick two-measurement mode and a six-measurement mode, in inches or cm, and compares US/Canada, UK, EU/France and Australia/NZ labels. It also gives sister sizes, a size chart, a measuring guide, a try-on fit checker and a fit card to copy, download or print.
An optional try-on journal saves only on your device, and measurements are processed in the browser, not uploaded. The site says results are sizes to try, not guarantees, and that the method is not clinically validated. It states it is independent of the A Bra That Fits project.
Shopping for a bra often starts with a chart that ignores sister sizes and how regional labels differ. Bra Size Calculator was built as a free starting point for trying sizes.