Lifestyle

Bra Size Calculator — Free 2026

Enter your band and bust measurements to find your bra size, cup size, and sister sizes instantly.

Band Measurement Measure around ribcage under bust

Bust Measurement Measure around fullest part of bust

Unit

Your Bra Size

—

Band Size

—

Cup Size

—

Full Size

Sister Sizes

How It Works

Enter measurements
View your size
Check sister sizes

Understanding Bra Sizing

Bra sizing uses two measurements: the band (underbust circumference) and the bust (fullest part of the chest). The band size is the underbust measurement rounded to the nearest even number. The cup size is determined by the difference between the bust and band measurements — each inch of difference corresponds to one cup size, starting at A for a 1-inch difference.

How the Calculation Works

The band measurement is rounded to the nearest even number to match standard band sizes (30, 32, 34, 36, etc.). The bust measurement is then compared to the band size. A 1-inch difference is an A cup, 2 inches is B, 3 is C, and so on through the alphabet. This tool supports cup sizes from AA (less than 1 inch) through K (11 inches). If you enter measurements in centimeters, they are converted to inches first since US bra sizing is inch-based.

Sister Sizes Explained

Sister sizes have equivalent cup volumes with different band sizes. When you go down a band size, you go up a cup size (and vice versa) to maintain the same volume. For example, 34C, 32D, and 36B are all sister sizes. This is useful when your regular size is not available or when a band feels too tight or loose. For more sizing tools, try our Dress Size Converter or Unit Converter.

Frequently Asked Questions

How do I measure my band size?

Wrap a measuring tape snugly around your ribcage, just under your bust. The tape should be level all the way around. Round to the nearest whole number. If the number is odd, round up to the next even number for your band size.

How do I measure my bust size?

Wrap the measuring tape around the fullest part of your bust. Keep the tape level and do not pull it too tight. Round to the nearest whole number.

What are sister sizes?

Sister sizes are bra sizes with a different band and cup combination that hold roughly the same cup volume. For example, if you wear 34C, your sister sizes are 32D (smaller band, bigger cup) and 36B (bigger band, smaller cup).

How accurate is this calculator?

This calculator provides a good starting point based on standard US sizing. However, bra fit can vary between brands and styles. We recommend trying on your calculated size and sister sizes to find the best fit.