Bra Size Calculator
Calculate your bra size from underbust and bust measurements with US, UK and EU conversions plus sister sizes.
Last reviewed by the Radiatus Cloud team
Measure over a non-padded bra with the tape level and snug. Sizing varies between brands — use the result as a starting point and check each brand's fit guide. Sister sizes share the same cup volume (band up = cup down).
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Calculate bra size from measurements
Bra sizing is genuinely confusing and varies between systems, and the right size is best derived from actual measurements rather than guesswork. This calculator computes your bra size from your underbust and bust measurements, and shows the result in US, UK and other systems so you can shop confidently wherever the brand is from.
Why measurement beats guessing
A bra size actually combines two separate things: a band size, which comes directly from the underbust measurement around the ribcage, and a cup size, which comes from the difference between the bust measurement and the underbust. Getting either of these wrong results in a poor, uncomfortable fit, and a great many people end up wearing the wrong size simply because they guessed at it rather than measuring. Calculating the size properly from the two measurements gives a correctly derived result, and because sizing systems differ noticeably between countries, showing several of them side by side is genuinely helpful when buying from different regions. It is a practical aid to finding a size that actually fits, which measurement-based sizing does far better than guesswork.
A calculation, not advice
It runs entirely in your browser, so nothing you enter is uploaded and it works without an account, keeping your figures on your own machine and available on any device immediately.
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Frequently Asked Questions
How is bra size calculated?
From two measurements: the underbust gives the band size, and the difference between bust and underbust gives the cup size.
Why do people wear the wrong size?
Often because they guess rather than measure. Calculating from measurements gives a properly derived size that fits better.
Why show different systems?
Because sizing differs between countries, so seeing US, UK and other systems helps when buying from different regions or brands.
What are the two measurements?
The underbust (around the ribcage under the bust) for the band, and the bust (around the fullest part), whose difference gives the cup.
Is my data uploaded?
No. The calculation runs entirely in your browser.
Privacy & Security
Runs entirely in your browser. No data is sent to any server.
How to Use
Measure your underbust and bust in inches or cm and enter both. Your size in US, UK and EU systems appears with sister-size alternatives.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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