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Bra Size Calculator

Two measurements — underbust and bust — give your size in US, UK, EU and French sizing, plus your sister sizes. Uses the measurement method, not the old “add 4 inches” rule that gets the band too big and the cup too small.

Why this is more accurate. Your band is your actual snug underbust measurement; your cup is the difference between your bust and that band — one cup per inch. We don't add inches to the band. US cup lettering above D varies by brand (DD vs E), so we show every system and your sister sizes to fine-tune the fit.

Enter your measurements

How to measure

  1. Underbust (band)

    Wrap the tape snugly around your ribcage directly under the bust, level all the way round. Breathe out. Or use our printable tape.

  2. Bust (cup)

    Measure around the fullest part of the bust, standing relaxed, tape level and not compressing.

  3. Read your size

    The calculator does band and cup for you and shows US, UK, EU and French sizes at once.

  4. Use sister sizes to fine-tune

    If the band feels off but the cup is right, move to a sister size — same cup volume, different band.

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How this is calculated

We use the measurement method, not the old +4/+5 rule. The band is your snug underbust measurement in inches (to the nearest even number); the cup is the difference between your standing bust measurement and the band, one cup per inch. US cup lettering above D varies by brand (DD vs E, DDD vs F), so treat letters as a guide and use sister sizes to fine-tune the band while keeping the same cup volume.

Sources: Measurement method per r/ABraThatFits calculator methodology (band = underbust, cup = bust − band); Standard international band scale (US/UK inches → EU/FR), cross-checked retailer charts 2026; Cup-letter divergence US vs UK vs EU documented on major lingerie size charts (2026).