How our tools work.
Calculations should be inspectable. This page describes the rules used by the tools, what their outputs mean and where chart data comes from.
The starting-size calculator
Convert centimetres to inches by dividing by 2.54. Round the snug underbust to the nearest even integer, with an exact odd-inch tie rounding upward. Subtract that band from the bust measurement, then round the difference to the nearest integer cup position.
Example: 34 inches underbust and 43 inches bust produce band 34 and a difference of 9. Position 9 gives UK G and US I. This is an independent two-measurement estimator. It is not the six-measurement A Bra That Fits algorithm and has not been clinically validated.
Supported underbust: 26–52 inches. Bust must exceed underbust and be at most 75 inches. Supported rounded cup differences: 1–15 inches. Inputs outside those bounds produce an explanation instead of an invented result. Measurement variation near rounding boundaries can change the estimate.
International conversion
The Freya international chart supplies label mappings for UK bands 28–44 and UK cups B–K. US/UK alignment is cross-checked against Wacoal’s Elomi, Freya and Fantasie chart. Conversions map labels, not body measurements. They do not guarantee identical fit between brands.
The Japanese explorer keeps Wacoal Japan body measurements separate from Freya export labels. Japanese domestic sizing must be read using the maker’s chart.
Sister sizes and comparison
One two-inch UK/US band step is paired with one opposite cup step for a nominal capacity match. The comparison tool accounts for both changes. This is a garment-grading relationship, not a breast volume, weight or percentage measurement.
Consult Bravissimo’s sister-size explanation and assess actual fit after changing bands.
Fit and construction checklists
These tools branch on observations and preferences. They suggest what to compare next; they do not diagnose anatomy, identify a guaranteed product or query a stock database. Wireless and wired centre-fit checks are handled differently.
Privacy and corrections
Measurement values are held only in the page and are not added to URLs or sent to an API. The clear action removes displayed values. Share a correction through our contact page without including personal measurements unless you choose to.
Chart research reviewed: 24 September 2026. We update the method when a source change or reproducible calculation error is identified.