Bra Size Calculator
A clothing-size tool. Enter your measurements and it works out a band and cup size, then converts that across UK, US, EU, AU, French and Japanese sizing and lists your sister sizes. Bra sizing has no international standard and no legal definition, so treat the result as a well-reasoned starting point for trying things on rather than a single correct answer.
Underbust measurements
Breathe out. Tape loosely under bust.
Comfortably snug under bust — used for band size.
As tight as comfortably possible under bust.
Bust measurements
Arms down, standing, at fullest point of bust.
Lean forward 90°, tape at fullest point.
Lie flat, tape at fullest point.
Your size will appear here
Enter your measurements and click Find my size to see your result plus sister sizes.
Band, Bust and the Difference Between Them
A bra size is two measurements and one subtraction. Once you see that, the rest of the system makes sense.
The band is your underbust, measured snugly, rounded to the nearest even number of inches. It carries most of the support, which is why it is the measurement that matters more of the two. Measuring loosely gives a band that is too big, and a band that is too big rides up and transfers the work to the straps.
The bust is measured at the fullest point. This calculator asks for it leaning forward as well as standing, because the two differ and the leaning figure is the one the cup has to accommodate.
The cup is the difference, not a size in itself. Subtract the band measurement from the bust: roughly one inch per cup letter. That is the part most people find counter-intuitive, because it means a cup letter has no fixed volume. A 30F and a 38F are nothing like the same cup — F simply means “about six inches more than the band”, and six inches around a 38 band encloses far more than six inches around a 30.
This is also why sister sizes work. Go down a band and up a cup, or up a band and down a cup, and the cup volume stays roughly the same while the fit around the ribcage changes. Those are listed with your result.
There Is No International Bra Size Standard
This is the single most useful thing to know, and it is why a calculator can only get you close.
Countries label cups differently. The same body measuring a 7-inch difference on a 32 band is a UK 32F, a US 32G, an EU 75G, a French 90G and an AU 16F. Nothing about the person changed. Above a D cup the UK and US systems diverge because the US inserts DDD where the UK uses E and F, so the gap widens the larger the cup.
Bands are counted differently too. UK and US bands are underbust inches; EU and Japanese bands are underbust centimetres rounded to the nearest 5; French bands are the EU number plus 15; Australian bands are the UK number minus 16. A “90” is a French band and an EU cup letter away from meaning anything similar.
Brands differ within a country. There is no legal definition of a cup size and no standards body enforcing one. Two brands in the same market, both labelling a 34C, can fit noticeably differently — and a single brand’s sizing often varies between styles. Anyone promising a perfect fit from measurements alone is overselling.
So use the result as a starting point: try that size, try both sister sizes, and let how it feels decide. A calculator narrows the search; it cannot end it.
The Same Size Across Six Systems
One set of measurements, converted. Every row is the same body.
| Measurements | UK | US | EU | FR | AU |
|---|---|---|---|---|---|
| 32″ band, 4″ difference | 32C | 32C | 80C | 95C | 16C |
| 32″ band, 6″ difference | 32DD | 32DD | 80E | 95E | 16DD |
| 32″ band, 7″ difference | 32F | 32G | 75G | 90G | 16F |
| 30″ band, 4.5″ difference | 30D | 30D | 70D | 85D | 14D |
Notice the third row: identical measurements, and the UK calls it F while the US calls it G. Below a D cup the two systems agree; above it they do not, which is exactly where people ordering across borders get caught out.
Quick Answer
Two numbers produce a bra size. Your band comes from your snug underbust measurement, rounded to the nearest even number of inches. Your cup comes from the difference between your bust and that underbust figure — roughly one inch of difference per cup letter, so a 7-inch difference is a larger cup than a 4-inch one. Everything else, including which letter that cup is called, depends on the sizing system and the brand. There is no international standard, which is why the same body is a UK 32F and a US 32G.
How It Works: Formula & Variables
Band size (UK/US) = snug underbust (in), rounded to nearest even number
Cup index = leaning bust (in) − snug underbust (in), rounded
EU band = snug underbust (cm), rounded to nearest 5
FR band = EU band + 15
AU band = UK band − 16
- Cup diff 1″
- A cup
- Cup diff 2″
- B cup
- Cup diff 3″
- C cup
- Cup diff 4″
- D cup
- Cup diff 5″
- DD (UK) / DD (US)
- Cup diff 6″
- E (UK) / DDD/F (US)
Worked Examples
Example 1: A 4-inch difference
Snug underbust 32″, bust 36″. The band rounds to 32, and the 4-inch difference gives a C cup: a UK and US 32C, an EU 80C, a French 95C, an AU 16C. Here the UK and US agree, because the systems only diverge above D.
Example 2: Where UK and US stop agreeing
Snug underbust 30.5″, bust 38″. The band rounds up to 32 and the 7.5-inch difference rounds to 7, giving a UK 32F — but a US 32G, an EU 75G and a French 90G. Ordering a “32F” from a US retailer would bring back a smaller cup than the UK label of the same name.
Example 3: Rounding at the half inch
Snug underbust 28.5″, bust 33″. The underbust rounds to 29, which is odd, so the band goes up to 30. The 4.5-inch difference rounds to 5 and then indexes to a D cup: UK and US 30D. Measurements landing on a half inch can tip either way, which is a good reason to try the neighbouring size too rather than trusting one decimal.
Example 4: Using sister sizes
Suppose a UK 32F cup fits but the band feels loose. The sister size down is 30FF — a tighter band with a cup of roughly the same volume, because dropping a band letter means going up a cup to compensate. If instead the band digs in, 34E goes the other way. Same volume, different fit around the ribcage.
Key Concepts
The +4 method is outdated: Older calculators added 4 inches to the underbust to get the band size — a workaround from an era when bras had limited stretch. Modern bras with elastic bands no longer need this adjustment. Using +4 pushes most people into a band that is too loose and a cup that is too small.
Cup size is relative, not absolute: A D cup on a 28 band is much smaller than a D cup on a 38 band. Cup letters only make sense alongside the band number — which is why sister sizes exist.
Your calculated size is a starting point: Sizing varies between brands and even between styles within the same brand. Always try on a bra and check the fit signs before buying.
Leaning bust is most accurate: When you lean forward, breast tissue falls fully away from the chest wall, giving a true volume reading. Standing bust tends to underestimate cup size for larger busts.
Common Mistakes
Measuring over clothing: Bras, padding, and thick fabric all add bulk. Measure on bare skin or a thin non-padded bra only.
Pulling the tape too tight on the bust: The bust tape should be parallel to the floor and loose enough to breathe comfortably — it should not compress the breast tissue.
Trusting in-store fitters who use the +4 method: Many chain-store fitters still use the outdated formula. If you've been consistently told you're a 34B or 36C and it never quite fits, recalculate with this tool.
Ignoring sister sizes: If a bra in your calculated size doesn't fit perfectly, try the sister sizes before dismissing the result — the band or cup shape may simply suit you better in the adjacent size.
Frequently Asked Questions
Most bra size calculators use only two measurements and apply an outdated "+4 inch" formula to the underbust — a workaround from an era when bras had little stretch. Bra fitting communities and independent research have consistently shown this produces the wrong size for the majority of people. Six measurements (three underbust, three bust) account for natural variation across body positions, giving a far more accurate result.
Sister sizes are bra sizes that share the same cup volume but use a different band size. For example, 34C, 36B, and 32D are all sister sizes — the actual cup is the same, only the band length changes. If your exact size is out of stock, a sister size will fit the cup equally well. Go up a band size and down a cup letter, or down a band size and up a cup letter.
The leaning bust measurement is generally the most accurate because gravity allows the breast tissue to hang fully forward, giving a true reading of the cup volume. This calculator uses your leaning bust as the primary measurement, falling back to standing bust if leaning is not entered.
Probably not. Studies estimate that 70–80% of people wear a band that is too large and a cup that is too small. Common sizes like 34B or 36C are over-manufactured and aggressively sold, so many people are fitted into them regardless of their actual measurements. Trust the numbers, try the suggested size on, and check the fit signs: the band should be level, the underwire should sit flat against your ribcage, and there should be no spillage.
There is no international standard for bra sizing. The US, UK, EU, Australian, French, and Japanese systems all use different scales for both the band and cup. The band number in the US and UK refers to inches, EU uses centimetres rounded to the nearest 5, and France adds 15 to the EU number. Cup letters also diverge after D — UK uses DD, E, F, FF, G while the US uses DD, DDD/F, G. This calculator converts your measurements into all major systems at once.
Yes. The 6-measurement method works for any chest shape, size, or gender. The calculator includes results for very small bands (28 and under) and very large cups (up to UK KK / US S) that traditional store calculators often ignore. If your size falls outside standard retail ranges, sister sizes or specialist lingerie brands are your best route.
Related Calculators
What this page is. A clothing-size tool. It converts measurements into a bra size and between sizing systems, and nothing more. It makes no assessment of health, body composition or anything else about you, and it should not be read as one. If something about your body has changed in a way that concerns you, that is a conversation for a doctor rather than a size chart.
Method and sources
- Method. Band from the snug underbust rounded to the nearest even inch; cup from the bust-minus-band difference at roughly one inch per letter. This follows the widely used A Bra That Fits methodology, a community-maintained approach rather than an official standard — because no official standard exists.
- No governing standard. Bra sizing is not covered by an international standard and cup letters have no legal definition. The conversion tables here reflect the conventions each market generally follows; individual brands deviate from them, some substantially.
- Conversions. UK and US bands are underbust inches; EU and Japanese bands are underbust centimetres rounded to the nearest 5; French bands are EU plus 15; Australian bands are UK minus 16. Cup letters follow each system's own sequence, which is why UK and US labels part company above D.
- Assumptions. Measurements in inches, taken level and snug but not compressing. Half-inch measurements can round either way and shift a size. Results are a starting point for fitting, not a guarantee, and no calculator can account for shape, style or a particular brand's cut.