Health & Fitness

Bra Size Calculator | Band, Cup & Sister Size in US, UK & EU

Calculate your bra size from underbust and bust measurements in inches or centimeters. Outputs your band and cup size in US, UK, EU/European, French, and Australian sizing with sister size recommendations and a fit checklist to verify the result matches your body.

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How to Measure

1. Band (underbust)

Measure snugly around your ribcage, directly under your bust. Keep the tape parallel to the floor. Round to the nearest whole number.

2. Bust measurement

For best accuracy: lean forward 90° and let breast tissue fall forward. Measure loosely around the fullest part. This is the "leaning" method used by professional fitters.

Underbust (inches)

Snug band measurement

Bust (inches)

Around fullest part

Bust Measurement Posture

How Bra Sizing Works

A bra size consists of a band number (the underbust measurement, rounded to the nearest even number) and a cup letter (the difference between bust and band measurement). Each inch of difference equals one cup size: AA=0, A=1, B=2, C=3, D=4, DD/E=5, and so on. Cup volume is relative to band size — a 34C and 36B hold the same breast volume (sister sizes).

Sister Sizes Explained

Sister sizes are bra sizes that share the same cup volume with a different band size. If you go up one band size, go down one cup letter to maintain the same fit. This is useful when your preferred size is unavailable: a 34D is a sister size to 32DD and 36C — all contain the same cup volume, just in different band lengths.

Cup Letter Naming by Country

Difference (in)	US/Canada	UK/Australia	EU (France)
4"	D	D	D
5"	DD	DD	E
6"	DDD/F	E	F
7"	G	F	G
8"	H	FF	H

Note: Bra sizing varies significantly between brands and styles. This calculator provides a starting estimate — try on several sizes around your calculated size, including sister sizes. For the most accurate fit, seek a professional fitting at a lingerie or department store.

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