What is your true bra size?
Bra sizing is more complicated than retailers make it appear, and the gap between label size and true fit is well documented in biomechanics research. Enter your measurements to find your true size and see how it compares to population data.
Querying population data…
Full bra size calculator
Combined band + cup with sister-size suggestions.
What is the average bra size?
The average bra size in the United States is currently 34DD, a significant shift from 34B recorded in 1992. This change reflects a combination of increasing average BMI across the population, as tracked in our weight percentile calculator, and improved awareness of proper fit - more women now seek professional fittings and buy sizes outside the narrow range historically stocked by mainstream retailers.
In the UK, surveys consistently report an average of 36DD. Both figures are meaningful baselines, but they describe what women are buying, not necessarily what they should be wearing. Research by McGhee and Steele (2010) found that around 80% of women are wearing a bra that does not match their measurements, with the most common error being too large a band and too small a cup.
Average bra size by country
Breast size averages vary considerably across populations, influenced by genetics, average BMI, and diet. Our breast asymmetry calculator shows that side-to-side size differences are the norm rather than the exception. The table below shows approximate national averages expressed in US sizing. Data drawn from Worlddata.info breast size dataset and published research.
| Country | Average band | Average cup (US) |
|---|---|---|
| USA | 34 | DD |
| UK | 36 | DD |
| Australia | 36 | D |
| Canada | 36 | C |
| France | 32 | B |
| Germany | 36 | C |
| Japan | 30 | A |
| Norway | 34 | D |
| Brazil | 40 | B |
| India | 34 | B |
Source: Worlddata.info breast size dataset. US cup equivalents used throughout for consistency.
Why 80% of women wear the wrong size
The landmark study by McGhee and Steele (2010), published in the Journal of Science and Medicine in Sport, measured 106 women and found that 70% were wearing a cup that was too small, and 10% had a band that was too large. In combination, only around 20% of participants were wearing a bra that matched their actual measurements.
Several factors drive this mismatch. Historically, retailers stocked a narrow range - primarily 32 to 38 in band, A to DD in cup - which forced many women into the nearest available size rather than their actual one. Fitting practices also varied widely, with some retailers using outdated methods that add several inches to the underbust measurement before selecting a band size. Modern evidence-based fitting starts from the actual underbust measurement rounded to the nearest even number, with no padding added.
Awareness has improved significantly since the 2010s, with a broader range of sizes now available online and in specialist retailers. Still, comfort and habit mean many women continue wearing a size they have worn for years regardless of whether it is the right fit.
How to measure your bra size at home
Accurate measuring requires a soft tape measure and a relaxed standing posture. Follow these steps:
Step 1 - Band size: Measure around your ribcage directly under your bust, keeping the tape snug but not tight. This is your underbust measurement in inches. Round to the nearest even number - this is your band size. If your underbust is 31 inches, your band is 32. If it is 33 inches, your band is 34.
Step 2 - Bust measurement: Measure around the fullest part of your bust, keeping the tape parallel to the floor. Do not pull tight - the tape should rest lightly against the skin.
Step 3 - Cup size: Subtract your band size from your bust measurement. Each inch of difference corresponds to a cup size: 0 inches = AA, 1 inch = A, 2 inches = B, 3 inches = C, 4 inches = D, 5 inches = DD, 6 inches = DDD, 7 inches = DDDD or E.
If you are measuring in centimetres, convert to inches first (divide by 2.54) before applying the formula, since cup sizing systems use inch-based differences.
What is sister sizing?
Sister sizes are bra sizes that contain the same cup volume as your true size, but with a different band and cup letter combination. Going down one band size and up one cup letter gives the same volume, as does going up one band size and down one cup letter.
For example, if your true size is 34C, your sister sizes are 32D (smaller band, larger cup) and 36B (larger band, smaller cup). The total breast volume in all three sizes is equivalent. Sister sizing is useful when your exact size is out of stock, or when a particular bra style runs tight or loose in the band.
| True size | Smaller band sister | Larger band sister |
|---|---|---|
| 34B | 32C | 36A |
| 34C | 32D | 36B |
| 34D | 32DD | 36C |
| 36C | 34D | 38B |
| 36DD | 34DDD | 38D |
Does breast size change with age?
A study by Coltman et al. (2017), published in Ergonomics, analysed 3D breast scan data and found that body mass index (BMI) is the primary driver of breast size variation, not age. Women with higher BMIs tend to have larger breast volumes regardless of age group. Age-related hormonal changes - particularly around the menopause - can alter breast shape and density, but the effect on volume is secondary to BMI changes.
Pregnancy and breastfeeding produce the most significant short-term changes in breast size, often shifting both band and cup measurement. Many women find their size changes permanently after pregnancy and do not return to pre-pregnancy measurements. It is worth remeasuring after any significant change in body weight or after pregnancy rather than assuming a previously worn size remains accurate.
Frequently asked questions
Not necessarily. This calculator uses an evidence-based formula from published research: the band is the underbust rounded to the nearest even number, and the cup is the difference in inches between bust and band. Some older fitting guides add extra inches to the underbust before selecting a band - this method often results in a larger band and smaller cup than the measurement-based approach used here. Try the calculated size if you have never done so, as many people find it a better fit than the size they have been buying.
Cup letter conventions vary. In the US and Canada, the sequence after D runs: DD, DDD, DDDD. In the UK and Australia, the same volumes are labelled: DD, E, F, FF, G. In France and some European countries, a different numeric band system is also used (add 15 to the underbust in cm to get the French band number). This calculator works in US sizing throughout and notes the country only for comparing your result to the national average, not for converting letter conventions.
The calculator will still produce a size from your measurements, but the result may not reflect how a bra will actually fit if you have had implants or a reduction. Implants in particular change the projection and shape of breast tissue in ways that affect how a standard cup fits. Professional fitting is strongly recommended after any breast surgery, as individual variation is high and a standard formula is less reliable in that context.
Breast size is not static. Weight changes of 5 kg or more typically shift both band and cup measurement noticeably. It is worth remeasuring after significant weight change, after pregnancy and breastfeeding, and after the menopause. Many fitting specialists recommend checking measurements annually as a general habit, since gradual weight changes and changes in posture can shift the fit of a previously accurate size over time.
In the US, the most commonly sold bra size is 34DD, according to retail data compiled by the International Journal of Fashion Design, Technology and Education (2020). In the UK, industry reports from Mintel (2023) put the most common size at 36DD. These figures have shifted upward over the past two decades. However, retail sales data reflects what women buy, not necessarily their correctly fitted size, so the true distribution may differ from purchase patterns.
A properly fitting bra has a band that sits level around the torso without riding up at the back, cups that fully contain the breast tissue without gapping or spillage, and straps that stay in place without digging in. McGhee and Steele (2010) in the Journal of Science and Medicine in Sport identified the most common fit errors as a band that is too loose and cups that are too small. A quick self-check is the "two-finger test": you should be able to fit two fingers under the band at the back, but not a full fist.
Several factors contribute. Rising average body weight is the primary driver: as BMI has increased across Western populations, so has breast volume. The CDC's NHANES data shows mean BMI for US women rose from 25.2 in 1980 to 30.5 by 2020. Better fitting education has also played a role, with more women moving into larger cup sizes that were previously underrepresented in retail. Coltman et al. (2017) in Ergonomics noted that improved measurement techniques and expanded size ranges have allowed more accurate sizing, which shifted reported averages upward.
Studies consistently find that between 70% and 85% of women wear an incorrectly fitted bra. McGhee and Steele (2010) found 80% of women in their sample wore the wrong size, most commonly a band too large and cup too small. A 2008 study by Greenbaum et al. in the Chiropractic and Osteopathy journal reported a similar figure of 70%. The most frequent error pattern is wearing a band size one or two sizes too large to compensate for a cup size that is too small, which reduces support and can contribute to shoulder and back discomfort.
- McGhee & Steele (2010). Journal of Science and Medicine in Sport - Incorrect bra fit
- White & Scurr (2012). Ergonomics - Breast measurement and sizing
- Coltman et al. (2017). Ergonomics - Three-dimensional breast scanning
- Worlddata.info breast size dataset