Bra Size Calculator

Find a source-based starting bra size

Enter snug underbust and fullest bust measurements in inches. Choose one published fitting method. US — Knix and UK — Brastop are independent starting references, not equivalent regional size conversions.

Source and sizing method

Snug underbust (inches)

Fullest bust (inches)

Knix US matches published underbust intervals and individual cup measurement anchors; gaps and between-anchor values are not rounded into an invented cup. Brastop UK requires a whole-inch underbust because its guide does not specify rounding for fractional measurements. Both methods are fitting starting points, not guaranteed sizes.

Select a source and enter two measurements to compare its published sizing reference.

Official references: Knix traditional bra-size chart and Brastop UK measuring guide (checked 2026-10-18). Sizemapped is not affiliated with either publisher. Bra fit also depends on shape, style and construction.

Diagram showing separate bust and underbust circumference measurement positions. — Take a snug underbust and a fullest-bust measurement separately before using the calculator.

Choose one published band-and-cup method

US Knix uses its traditional bra measurement chart: find the published underbust interval, then compare fullest bust with the individual cup anchors in that band column. The largest bands have a half-inch shift in those anchors; an all-purpose bust-minus-band formula would lose this detail.

UK Brastop follows its own tape-measure guide: an odd whole-inch underbust rounds up to the next even band, then the published A–K cup sequence uses rounded fullest bust minus the originally measured underbust. These are independent starting references, not direct US-to-UK bra-size equivalences.

Gaps and between-row measurements

Knix does not publish a continuous underbust interval for every decimal measurement. If your underbust is between its published ranges, the tool explains the gap rather than silently choosing a band. A fullest-bust value between two cup anchors shows neighboring size options instead of inventing a half-cup.

Brastop does not explicitly specify rounding fractional underbust values, so this tool asks for a whole-inch underbust in UK mode rather than inferring a rounding policy. Both sources say fit, shape and model construction matter after any number is calculated.

Worked example: 30-inch underbust and 35-inch bust

Knix US chart: 30 inches falls within the published 29–30.75-inch underbust interval (band 34); the 35-inch fullest-bust anchor is B, giving a Knix US starting reference of 34B.

Brastop UK method: 30-inch underbust gives band 30; 35 minus the measured 30 is 5 inches, giving UK DD, or 30DD. These results are not asserted to be interchangeable retail sizes.

Method & references

How this guidance is checked

These sources support the measurement method and help check real-world variation. They are not presented as one universal brand-size table.

Bra Fit Calculator — Wacoal. Bra measurement cross-check using underbust and fullest-bust measurements and brand-specific fitting logic.

Common questions

Frequently asked questions

Why do Knix US and Brastop UK give different labels?

They use different published band and cup calculation methods. Their results are independent fitting references, not equivalent sizes across regions.

Can I round an unlisted decimal measurement to the nearest band?

Not automatically. Knix has published underbust gaps and Brastop does not state a fractional underbust rounding rule. Recheck the measurement or consult the linked publisher chart.

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