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Bra Size Converter

Estimate bra size from underbust (band) and bust circumference, with a measuring guide and cross-chart labels. Shape and stretch still matter — use this as a starting point.

Calculate

Measure underbust firmly, then bust loosely at the fullest point 1 UNDERBUST firm · under breasts 2 BUST loose · fullest part keep the tape level all the way around cup ≈ bust − underbust
How to measure for a bra →

Updates as you type

Result

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The formula

US band is underbust rounded to an even inch. Cup letter comes from bust minus underbust (about one letter per inch). EU/FR bands are underbust cm rounded to 5.

band ≈ round_even(underbust) · cup ≈ bust − underbust (inches)

How to measure for a bra

Wear a light bra or none; keep the tape level.

Measure underbust firmly, then bust loosely at the fullest point 1 UNDERBUST firm · under breasts 2 BUST loose · fullest part keep the tape level all the way around cup ≈ bust − underbust
  1. Underbust — Measure firmly under the bust.
  2. Bust — Measure loosely around the fullest part.
  3. Convert — Cup comes from the difference.
  • Sister sizes exist such as 34C and 36B.
  • Sports styles may need a different cup.

Worked example

  1. Underbust 34 in, bust 37 in
  2. Band → 34 · difference = 3 in → cup ≈ C
  3. US/UK ≈ 34C · EU band ≈ 85 with matching cup letter

Result: About 34C (US/UK)

How bra size is estimated

Traditional US fitting uses two circumferences: a firm underbust for the band and a fuller bust for the cup. Cup volume is the difference between them.

Band and cup

Band inches are rounded to an even number (minimum 28 in our chart). Cup index increases roughly one letter per inch of difference (AA, A, B, C, D, DD/E…). Letters diverge across regions after D.

Units

Enter inches or centimetres. Centimetre inputs are converted to inches before the US method runs; EU labels show a centimetre band.

Sister sizes

Going up a band and down a cup (34C ↔ 36B) keeps similar cup volume with a different band tension. Try sister sizes when the cup fits but the band does not.

What we do not model

No shape (projected vs shallow), wire width, or sports compression. Brands grade differently — especially beyond DD.

Interesting facts

Band does the lifting

Most support comes from the band, not the straps — a snug band is the foundation.

EU bands in cm

EU/FR often label the underbust in centimetres (75, 80, 85) instead of UK/US inches.

Sister sizes

Going up a band and down a cup (34C ↔ 36B) keeps similar cup volume with a different band.

UK vs US cups

Cup letters diverge after D — UK uses DD, E, F while US often uses DD/E, DDD/F.

Measure at day’s end

The ribcage expands slightly during the day; evening measurements better match all-day wear.

Frequently asked questions

Measure underbust and fullest bust. Band ≈ even underbust inches; cup ≈ inch difference. Enter both values here to see US/UK and EU-style labels.

Pick the unit that matches your tape. We convert cm to inches internally for the US method.

Pairs like 34C and 36B that keep similar cup volume while changing band. Useful when cups fit but the band rides up or feels tight.

Shape, stretch, and grading differ. Treat this as a starting point and use brand charts plus returns.

Yes for this method — if bust ≤ underbust, check tape placement or measuring error; the calculator will not return a size.

References

  1. ISO 8559 — Size designation of clothes — International Organization for Standardization International clothing measurement vocabulary.
  2. NIST — length units — National Institute of Standards and Technology Exact cm/inch conversion factors.
  3. Breast health information — U.S. Centers for Disease Control and Prevention (CDC) General breast-health context; sizing tools are not medical devices.