How do I measure underbust correctly?

Wrap a soft measuring tape firmly (but not tightly) around your ribcage directly below your bust. Keep the tape parallel to the floor. Breathe normally and note the measurement in inches or centimeters.

How do I measure overbust correctly?

Wrap the tape loosely around the fullest part of your bust, keeping it parallel to the floor. Do not compress the breast tissue — the tape should just make contact all the way around.

What is a sister size?

Sister sizes are bra sizes that contain the same cup volume as your calculated size but with a different band size. Going one band size up and one cup letter down (or vice versa) gives the same interior volume. This is useful when a preferred band size is out of stock or for a different fit feel.

Why might my calculated size differ from my usual size?

This tool uses the modern measuring method: the band is your underbust rounded to the nearest inch (odd numbers go up to the next even band), and each inch of difference between bust and band is one cup. Older charts add 4 or 5 inches to the underbust, which gives a band that is too large and a cup that is too small. Brands still vary, so try a bra on when you can.

What does AA, A, B, C, D, DD mean?

The cup letter is the difference between your bust and band, rounded to the nearest inch: under 1 in = AA, 1 in = A, 2 in = B, 3 in = C, 4 in = D, 5 in = DD (E), 6 in = DDD (F), 7 in = G, 8 in = H. Each cup step is one inch.

Bra Size Calculator

Enter your underbust and overbust measurements to find your US bra size and explore sister sizes.

Enter Your Measurements

Unit

Underbust (below the bust, snug)

Overbust (fullest part, loose)

How to Measure

Underbust: Tape snugly around ribcage just under the bust. Note the number while breathing normally.
Overbust: Tape loosely around the fullest part of the bust, parallel to the floor.

Summary

This bra size calculator converts your underbust and overbust measurements into a US bra size — band size plus cup letter. Enter measurements in inches or centimeters, and the tool handles the conversion and rounding automatically. It also shows a sister size chart so you can find alternative band/cup combinations that offer the same fit volume.

How it works

Choose your preferred unit: inches or centimeters.
Measure your underbust (snugly around the ribcage just below the bust) and enter the value.
Measure your overbust (loosely around the fullest part of the bust) and enter the value.
Your calculated band size and cup size appear instantly.
Review the sister size chart to find equivalent fits with different band sizes.

Use cases

Finding your correct bra size before shopping online.
Checking if a different band/cup combination will fit as well (sister sizing).
Converting measurements from cm to the US sizing system.
Verifying your size after weight change or pregnancy.
Helping a friend or family member find their bra size as a gift guide.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu