Bra Size Calculator
Estimates band and cup bra size from underbust and bust measurements, including sister sizes.
Your measurements
Measure snugly directly under the bust, around the ribcage. Used for the band size.
Measure around the fullest part of the bust, tape level and not tight. Used for the cup size.
Measurements are processed only in your browser and never stored or sent anywhere. If you need help measuring, ask a trusted parent/guardian.
Your estimated bra size
Enter your underbust and bust measurements to estimate a band and cup size. Bra sizing varies a lot by brand and style.
Before you rely on this: Use this as a quick guide and check the result against your own working before you rely on it.
How to use this calculator
- Choose cm or inches.
- Measure your underbust (under the bust, around the ribcage) for the band.
- Measure your bust (fullest part) for the cup, then press Estimate bra size.
How it works
The Bra Size Calculator estimates an AU/UK band and cup size from your underbust and bust measurements, and suggests sister sizes to try.
The band comes from the underbust measurement; the cup comes from the difference between bust and underbust (roughly one cup per 2.5 cm). Because band firmness, cup shape and brand vary, it also shows sister sizes (one band down/cup up and one band up/cup down).
Tips
- Measure in a non-padded bra or unclothed, keeping the tape level.
- If the band rides up at the back, try the sister size with a firmer band.
Frequently asked questions
What are sister sizes?
Sister sizes have a similar cup volume with a different band — for example going one band down and one cup up. They are useful when the band feels too tight or loose but the cup is close.
Are AU/UK bra sizes the same as US or EU?
No. Band and cup labelling differs by system, especially above a D cup. Use a brand conversion chart when shopping internationally.
Is my measurement data stored?
No. Everything is calculated in your browser and nothing is saved or sent anywhere.
How accurate are these size results?
They are a helpful starting point, not a guarantee. Brand sizing, garment cut, fabric stretch, body shape and measurement technique all change the best fit, so we show a recommended size plus adjacent sizes rather than one exact answer.
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Cite this tool
Zerdly. (2026). Bra Size Calculator. Retrieved from https://www.zerdly.com/tools/bra-size-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.