Bra Size Calculator

Estimate a starting bra size from snug underbust and full-bust measurements, with common UK, US, EU, French/Spanish and Italian conversions.

How to measure: Band — measure snugly around your ribcage directly under your bust. Bust — measure loosely around the fullest part of your chest.

Band measurement (inches)

Bust measurement (inches)

Brand grading and cup naming vary. Treat every result as a fitting starting point and check the manufacturer’s chart.

About This Tool

What it does

Rounds the snug underbust to a supported UK/US band and estimates cup volume from the difference between full bust and rounded band. It then maps the same cup volume to common regional labels.

Who it's for

Adults seeking a starting size for fitting. Bra sizing is not medically standardised, and brand construction, breast shape and personal fit preferences can change the best size.

Your privacy

All calculations happen in your browser. No measurements or personal data are stored or transmitted anywhere.

Why conversions are approximate

Band numbering and cup sequences differ by region and manufacturer. This calculator uses a common UK progression and maps cup volume to common US and continental labels, but a brand chart and try-on remain the authority for a particular garment.

Sizing references

Reviewed 23 June 2026: ISO 8559-1: body measurements; Wacoal size charts; Triumph international size charts.

Frequently Asked Questions

How is the starting size estimated?

The snug underbust is rounded to a supported even UK/US band. The full-bust difference from that rounded band determines a common UK cup-volume step.

Why do UK and US cups diverge?

The systems use different letter progressions above D. The calculator maps equivalent cup volume, but manufacturers may label larger cups differently.

How are EU, French and Italian bands converted?

Common conversions map UK 30 to EU 65, French/Spanish 80 and Italian 1. These are regional references; always check the brand chart.

What are sister sizes?

Sister sizes keep approximately the same cup volume while moving one band step down and one cup step up, or one band step up and one cup step down.

Is the calculated size guaranteed to fit?

No. It is a starting point. Brand grading, wire shape, fabric, breast shape and personal comfort can all change the best-fitting size.