Bra Size Calculator
Bra size calculator: your bra size from two measurements, with sister sizes and US, UK, EU, French and AU sizes. Free, no sign-up, and it runs in your browser.
Dedicated page: /en/bra-size-calculator
Estimate only, not medical advice. This is a starting size to try first: brands cut differently, so always check the brand's own chart before ordering. Measurements stay in your browser and are never uploaded.
Snug around the ribcage, just under the bust, tape level, after breathing out.
Loosely around the fullest part, tape level, without squashing.
Modern method, no plus 4: the band comes straight from your underbust, rounded to the nearest even number. The old plus-4 rule is from the 1930s and gives bands several sizes too loose for modern stretch fabric.
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Quick answer
A bra size calculator works out your size from two measurements: the underbust, which sets the band, and the bust, which sets the cup. The band is your underbust rounded to the nearest even number, with nothing added, and the cup is one letter per inch of difference between bust and band.
About this Bra Size Calculator
Bra size calculator: your bra size from two measurements, with sister sizes and US, UK, EU, French and AU sizes. It is one of the Health and Family on ToolUstad, and like every tool here it runs entirely in your browser: nothing you type, paste or open is uploaded to a server, and there is no sign-up or usage limit.
What is a bra size calculator?
A bra size calculator turns two measurements into the size to try first. The underbust, measured snugly around the ribcage just under the bust, sets the band number. The bust, measured loosely around the fullest part, sets the cup letter: subtract the band from the bust, and each inch of difference is one cup step. One inch is an A cup, two inches a B, three a C, four a D, five a DD, six a DDD, and so on. A 34-inch underbust with a 37-inch bust is a 34C in US sizing. The same measurements are a 75C in EU sizing and a 12C in Australian sizing.
The cup letter on its own means nothing, because a cup is a volume that depends on the band. A 30D and a 38D hold completely different volumes, because the bands they sit on are four sizes apart.
The modern method, and why plus 4 is wrong
Many older calculators tell you to add 4 inches to your underbust to get your band. That rule comes from the 1930s, when bra fabric had no stretch and bands needed extra slack to breathe. Modern bras use elastic blends, and adding 4 inches gives a band two to four sizes too loose. A loose band rides up the back, pushes weight onto the shoulders, and makes the cup read wrong. The modern method, used by professional fitters, takes the underbust as it is and rounds it to the nearest even number.
US, UK and EU sizing
Band numbers match between the US and the UK: a 34 is a 34 in both. Cups match up to and including D and DD, then they split. US sizing continues DD, DDD, sometimes written F, then G, H. UK sizing goes DD, E, F, FF, G, GG, so a US DDD is a UK F, a US G is a UK FF, and the UK E sits between the US DD and DDD with no US equivalent letter at all.
Best practices
- Measure in the morning, in a non-padded bra: the band sits flat, the tape stays level, and the numbers are at their most stable.
- Round the underbust to the nearest even number: bands only come in even numbers, and a smaller band stretches with wear.
Common mistakes to avoid
- Adding 4 inches to the underbust, which gives a band several sizes too loose for modern stretch fabric.
- Changing the cup when the band is the problem: if the band rides up, the fix is a sister size, not a different cup.
Who is the bra size calculator for?
Anyone buying a bra online, anyone whose size has changed after weight change or pregnancy, and anyone whose current bra rides up, gapes, or digs in and wants to know which size to try next.
Related terms: bra size chart, how to measure bra size, sister sizes, US to UK bra size, EU bra size conversion, cup size chart, band size.
Estimate only, not medical advice. This is a starting size to try first: brands cut differently, so check the brand's own chart before ordering.
Learn: how to use the Bra Size Calculator
- Measure your underbust snugly, just under the bust, and your bust loosely at the fullest point.
- Enter both numbers in inches or centimetres.
- Read your US size, the conversions, the sister sizes, and the fix if something does not fit.
Features
- Modern no-plus-4 method: band straight from your underbust
- Band rounded to the nearest even number, cups one letter per inch
- Sister sizes up and down, with when to use each
- US, UK, EU, French and Australian conversions in one view
- Full cup chart from AA to K across US, UK and EU
- Fit troubleshooter for five common problems
- Inches and centimetres, with conversion done inside the tool
Worked example
Underbust 34 inches and bust 37 inches: the band is 34, the difference is 3 inches, and 3 inches means a C cup, so the size is 34C in the US. Sister sizes are 36B if the band feels tight and 32D if the band rides up loose. The same measurements convert to 75C in EU sizing and 12C in Australian sizing.
Frequently asked questions
How do I measure my bra size at home?
Wear a non-padded bra or none. Measure snugly around your ribcage just under the bust, tape level, after breathing out: that is your underbust. Then measure loosely around the fullest part of your bust. Enter both numbers and the calculator does the rest.
Should I add 4 inches to my underbust?
No. The plus-4 rule is from the 1930s, when bra fabric had no stretch and bands needed extra slack. Modern bras use elastic, and adding 4 inches gives a band several sizes too loose, which then makes the cup wrong too.
What are sister sizes?
Sizes that hold the same cup volume on a different band. Go one band up and one cup down, or one band down and one cup up: 34C, 32D and 36B are sister sizes.
Why does the same size fit differently between brands?
Brands cut bands and cups differently, grade their own way above a D cup, and use different wire widths and strap placements. A 34B from one brand is not identical to a 34B from another. Treat the calculator result as the size to try first.
What is 34B in EU size?
A US or UK 34B is a EU 75B, a French 90B, an Australian 12B, an Italian 3B and a Japanese 75B. The band number changes because Europe labels the band in centimetres, 75 cm, while the US and UK label it in inches, 34 in.
How do US and UK cup sizes differ?
They match up to and including D and DD, then they split. US sizing goes DD, DDD, G, H. UK sizing goes DD, E, F, FF, G, GG. So a US DDD is a UK F, and a US G is a UK FF.
How often should I re-measure?
Every 6 to 12 months, or after any weight change, pregnancy, weaning, or a hormonal change. Bodies change, and a size from two years ago is a guess, not a measurement.
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