So many women are wearing the wrong size bra, and the high street gives very few options to get measured properly to fit modern bras. This calculator uses our algorithm to offer a starting size to then try bras. Using this, and our fit guides, you can try on bras knowing what should be happening to get your best fit. How To Measure Skip straight to calculator. Please enter your measurements below to calculate your Boob or Bust recommended Starting Size. How to measure.