Free bra size calculator: enter your underbust and bust in inches or cm to find your band, cup, sister sizes, and US, UK, EU and FR conversions.
Enter your underbust and bust measurements to find your size.
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Enter two measurements — your underbust (band) and your bust — in inches or centimeters, and this calculator finds your band and cup size, your equivalent sizes in the US, UK, EU, FR and AU/NZ systems, and your sister sizes. It uses the modern snug-underbust method that fits today's bras, runs entirely in your browser, and treats every size the same. Use it as an accurate starting point, then confirm the fit by trying bras on.
A bra size has two parts. The band (the number, like 34) comes from your underbust measurement — the snug measurement around your ribcage just under the bust. The cup (the letter, like C) comes from the difference between your bust and your band: each inch of difference is one cup size, so a 1-inch difference is an A, 2 inches a B, 3 inches a C, and so on. Older charts told you to add 4 inches to the band, but modern bras fit best measured directly, which is why this calculator uses the snug-underbust method by default.
Cup size
Know your size before buying and convert it for international brands.
Try a sister size when the band feels too tight or too loose.
Re-measure after weight changes, pregnancy, or simply over time.
Convert a known size to the retailer's sizing system.
A simple, private way to find a starting size at home.
Translate your size across US, UK, EU, FR and AU/NZ labels.
Uses your snug underbust directly instead of the outdated +4 inches, which fits today's bras far better and is why most people are in the wrong size.
See your equivalent size across the major systems at once — handy for shopping international brands where cup letters differ.
Get the same cup volume in a snugger or looser band, so you have options when the band feels off.
Measure in whichever unit you have; the calculator handles the conversion accurately.
Every size is treated equally, with simple measuring steps and honest fit tips.
Everything runs in your browser. Your measurements are never stored or shared, and there is no sign-up.
Measure your band snugly around your ribcage directly under your bust, then measure around the fullest part of your bust. Enter both in inches or centimeters and the calculator does the rest.
Cup size is the difference between your bust and band measurements: each inch of difference is one cup size. A 1-inch difference is an A, 2 inches a B, 3 inches a C, 4 inches a D, and so on.
Usually no. The modern method uses your snug underbust directly. The old +4 method tends to give a band that is too loose on modern bras. Use the Recommended method unless a specific brand's guide asks for +4.
Sister sizes have the same cup volume but a different band — for example 34C, 32D and 36B. If your band feels too tight or too loose, a sister size gives a similar cup with a band that fits better.
The calculator shows US, UK, EU, FR and AU/NZ sizes together. Cup letters diverge above a D cup, so use the converted letter for each system rather than assuming the US letter applies everywhere.
That usually means the two measurements were entered in the wrong fields, or the band tape was not snug. Re-measure with the band tape firm and the bust tape relaxed, and check the fields.
Sizing varies a lot by brand and style. Use this as a starting point, check the fit (band level and snug, cups filled without spilling or gaping), and try a sister size if the band or cup feels off.
Yes. Select Centimeters and enter both measurements in cm — the calculator converts internally and returns the same result as inches.