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Redmoon Converters
🌿 Health & Lifestyle

Bra Size Converter (UK / US / EU / AU / JP)

Convert bra sizes between UK, US, EU, AU and Japanese systems, plus "sister size" recommendations when the band or cup doesn't fit perfectly.

Built and maintained by Paul Clark, Redmoon Software · Checked against what this tool computes on

Estimated size
—
Band: — cm · Cup difference: — cm
SystemSize
Sister sizes (same cup volume, ± band)
USUKEUJP
Quick reference

US/UK bands are the underbust in inches rounded up to an even number. EU and JP bands use the underbust in centimetres, rounded to the nearest 5. Australian bands run 20 below US/UK, so a US 34 is an AU 14.

The cup letter comes from the difference between your bust measurement and your band size, one letter per inch: under 1 in is AA, 1 in is A, 2 in B, 3 in C, 4 in D, 5 in DD, and so on up. That is the convention every mainstream US and UK brand chart uses. Above DD the two systems diverge slightly — US calls the next size DDD/E where the UK calls it E — so the table shows each in its own notation.

Cup letters are not absolute volumes: 34D, 32DD and 36C all hold roughly the same cup. Those are sister sizes, listed below the result.

Sister sizing: drop one band → go up one cup (and vice versa) keeps the same cup volume. Useful when the band fits but the cup doesn't, or the other way around.

How it works

Work from two measurements. Underbust and bust are converted to centimetres, with inches multiplied by 2.54, and the band is then the underbust in inches rounded up to the next even number — the modern snug-band method, with no +4 or +5 added. EU and JP bands are the underbust in centimetres rounded to the nearest 5. The cup letter comes from bust minus underbust, read off a 2.5 cm per letter ladder that this implementation starts at AA = 10 cm.

Input mode switches between Measurements, defaulting to 75 cm underbust and 91 cm bust, and Existing size, where typing 34D and choosing a system runs the same arithmetic backwards to recover measurements. Measurement unit applies only in the first mode. The sister-size table is the part most worth using: it steps two bands either side of your result and moves the cup letter the opposite way each step, because dropping a band and gaining a cup holds roughly the same cup volume.

Treat the letter as a starting point rather than a fitting. The cup ladder used here is a metric one, 2.5 cm per step anchored at AA for a 10 cm difference, which resembles how JP and EU cups are built but sits several letters away from the US and UK convention of one cup per inch over the band, so the US and UK letters shown read small. Cup letters are not standardised between brands anyway, and nothing here captures breast shape or root width.

Frequently asked questions

How do I measure my band and bust at home?

Measure snugly around your ribcage directly under the bust for the underbust figure, then around the fullest part of the bust for the second. Enter both in the same unit, with inches converted at 2.54 cm. The 75 and 91 cm defaults show the expected format.

What is a sister size and when should I use one?

Sister sizes hold cup volume roughly constant while shifting the band. Drop one band and go up one cup, or the reverse, which is what the table steps through two sizes either way. Use it when the band digs in but the cups fit, or the band rides up.

Is a UK bra size the same as a US size?

This tool prints the same band number and the same cup letter for both. In practice UK sizing runs DD, E, F, FF, G where US often uses DD, DDD, F, so above a D cup the two systems diverge and the brand's own chart should be checked.

How is the Australian size worked out here?

The AU band is the US band minus 14, so a US 34 prints as AU 20, and the cup letter passes through unchanged. Australian charts commonly map a US or UK 34 to AU 14 instead, so verify against the brand's chart before ordering.

Why does entering 34E give a strange result?

The size parser matches a fixed ladder of AA, A, B, C, D, DD, DDD/E, F, FF, G, GG, H, HH and J. A bare E is not on that list and silently falls back to AA. Enter DDD for that size, or use Measurements mode instead.

Cup letters are relative, not absolute

A cup size is the difference between bust and underbust measurement, not a fixed volume. That means a 32D and a 38D have the same four-inch difference and completely different cup volumes — sizes are not comparable across bands, which is the single most misunderstood thing about the system.

Sister sizing follows from that: going down a band and up a cup, or the reverse, keeps approximate cup volume while changing the band. It is what makes a size that is nearly right adjustable.

Why conversions between systems are approximate

UK, US, EU and Australian systems use different band numbering and different cup progressions, and they diverge further at larger cup sizes where some systems use doubled letters and others continue the alphabet. A conversion table gets you to a starting point rather than an equivalent.

Manufacturer variation is at least as large as system variation. Two garments labelled identically from different brands frequently fit differently, and cut differs by style within a brand.

The band should carry most of the support and should be firm on the loosest hook when new, since bands stretch with wear. Measurement gets you a candidate size; fit is decided by trying it.

Where these numbers come from

The standard or formula behind this tool, so you can check it rather than take it on trust.

  • UK, US, EU and AU sizing systems use different band and cup step conventions.

    Sister sizing means several labelled sizes share a cup volume, so a conversion is a set of candidates rather than one answer.

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