What are sister sizes?

Sister sizes are bras with different band numbers but a similar cup volume. Moving down one band size requires going up one cup letter (and vice versa), so 32DDD, 34DD, and 30G hold a comparable amount.

Should I round my band measurement up?

This calculator rounds the band to the nearest even number rather than always rounding up. If your number falls between sizes and the band feels tight, try the next size up using the sister-size suggestions.

Why does each inch of difference change the cup letter?

Cup size reflects how much the bust projects beyond the ribcage, so each additional inch of bust-minus-band difference corresponds to one larger cup in this standard US sizing scheme.

Do bra sizes vary between brands?

Yes. This gives a solid starting size, but cut and sizing differ by brand and style, so always try the calculated size plus its sister sizes when shopping.

Bra Size Calculator

Band Measurement

Bust Measurement

US Size34B

Sister Sizes36A, 32C

This bra size calculator turns two tape measurements into a US bra size and a set of sister sizes. The band (underbust) measurement sets the numeric band size, and the difference between the bust (fullest part) and the band sets the cup letter. It also suggests sister sizes, which are alternate band-and-cup combinations that share roughly the same cup volume for when the band feels too tight or too loose.

Formula

bandSize = round(band/2)×2; cupIndex = round(bust − bandSize)

band: Underbust measurement in inches
bust: Fullest-bust measurement in inches
cupIndex: Inch difference mapped to a cup letter (0=AA, 1=A, 2=B, 3=C, 4=D, 5=DD, 6=DDD...)

How it works

Measure snugly around your ribcage just under the bust for the band, and around the fullest part of the bust for the bust measurement, both in inches.
The band measurement is rounded to the nearest even number to give the band size, and the bust-minus-band difference (in whole inches) selects the cup: 0=AA, 1=A, 2=B, 3=C, 4=D, 5=DD, 6=DDD, and so on.
The result shows the combined US size and up to two sister sizes — going down a band adds a cup letter, going up a band drops one — staying within a 28-48 band range.

Worked example

A band measurement of 32 inches and a bust measurement of 38 inches.

Band size: round(32/2)×2 = 32.
Difference: 38 − 32 = 6 inches, which maps to the DDD cup.
Sister sizes: one band up becomes 34DD, one band down becomes 30G.

32DDD, with sister sizes 34DD and 30G.

Frequently asked questions

What are sister sizes?: Sister sizes are bras with different band numbers but a similar cup volume. Moving down one band size requires going up one cup letter (and vice versa), so 32DDD, 34DD, and 30G hold a comparable amount.
Should I round my band measurement up?: This calculator rounds the band to the nearest even number rather than always rounding up. If your number falls between sizes and the band feels tight, try the next size up using the sister-size suggestions.
Why does each inch of difference change the cup letter?: Cup size reflects how much the bust projects beyond the ribcage, so each additional inch of bust-minus-band difference corresponds to one larger cup in this standard US sizing scheme.
Do bra sizes vary between brands?: Yes. This gives a solid starting size, but cut and sizing differ by brand and style, so always try the calculated size plus its sister sizes when shopping.

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