Bra Size Calculator
Calculate your accurate bra band and cup size using underbust and bust measurements in inches or cm.
Input Details
Calculated from 30" underbust and 34" bust (4" bust-band difference).
About Bra Size Calculator
Calculate your true bra band and cup size using underbust and bust measurements.
How to Use
- Measure around your ribs directly under your bust (underbust measurement).
- Measure around the fullest part of your bust.
- Enter measurements in inches or centimeters.
- View your estimated band size, cup size, and US/UK/EU conversions.
Formula
Band = Underbust Measurement Rounded to the Nearest Even Number | Cup = Bust − Band Difference
This calculator uses the direct-measurement method: your underbust measurement is rounded to the nearest even number to get your band size (it does not add 4 inches, unlike the older method some stores still use). Cup size is then based on the difference between your bust and underbust measurement (1" = A, 2" = B, 3" = C, 4" = D, 5" = DD/E).
Example
30" underbust + 34" bust = 30 Band with 4" difference (D Cup) = 30D.
Important Notes
- This tool offers a mathematical fitting estimate. Bra shape, wire type, and fabric stretch affect actual fit.
Frequently Asked Questions
How do I calculate my bra cup size?
Subtract your snug underbust measurement from your full bust measurement. Each 1 inch of difference equals one cup size letter.
Why does my calculated bra size differ from what I wear?
Many stores use outdated +4 band addition rules or vanity sizing. Sister sizing allows different band/cup combinations to fit similarly.
What is sister sizing in bras?
Sister sizes have the same cup volume. If you go up a band size, go down a cup size (e.g. 32D, 34C, and 36B hold the same cup volume).