What is your true bra size, and how does it compare?
Most women learn their bra size from a single measurement taken years ago. Sizing standards differ between countries, and the average size has shifted significantly over the past three decades. Enter your measurements to find out where you actually sit.
Querying population data…
Full bra size calculator
Combined band + cup with sister-size suggestions.
What is the average bra size? US and UK data
The average bra size in the United States is 34DD, a significant increase from the average of 34B in 1992. In the UK, the average is 36DD, also having risen from a C cup in the 1990s. These figures come from retail sales data, industry surveys, and fitting studies, though they carry important caveats: average size is derived from what people buy, not from professional measurement of what they should be wearing — and research consistently shows these are not the same thing.
The increase in average cup size over the past three decades reflects a combination of genuine change (higher average BMI, which correlates strongly with breast volume) and methodological change (greater awareness of correct fitting, which tends to move women from underbanded and under-cupped sizes into larger cups). Breast volume correlates significantly with BMI: Coltman, Steele, and McGhee (2017) found average breast volume of approximately 490-520ml per breast for women with BMI 18.5-24.9, rising to 850ml+ at BMI 30+. As average BMI has increased in both the US and UK, average breast size has followed. The average cup size has also shifted as women have become more aware of the bra fit problem described in the section below.
The most common bra size varies by country. In the US and UK, 34DD/36DD are now the modal sizes in retail sales. Countries with lower average BMI (Japan, South Korea, much of Southeast Asia) have significantly smaller average cup sizes. Norway has the largest documented average cup size in Europe. These cross-national differences are primarily driven by BMI differences rather than genetic variation in breast anatomy independent of body size.
Why 80% of women wear the wrong bra size
The most widely cited statistic in bra fitting is that approximately 80-85% of women wear an incorrectly fitted bra. This comes from McGhee and Steele (2010, Journal of Science and Medicine in Sport, n=104 women), who found that 70% were wearing a band and/or cup that was too small and 10% were wearing one too large. A subsequent study by White and Scurr (2012, Ergonomics) at the University of Portsmouth found that even professional fitters produced inconsistent results — trained fitters disagreed on correct size for the same woman across assessments.
The root cause is the measuring system itself. The traditional "+4 inches" method (adding 4 inches to the underbust measurement to get band size) was developed for the stiff fabrics of early bra construction and systematically produces bands that are too loose and cups that are therefore too small. A woman who measures 30 inches under the bust would traditionally be told she is a 34A or 34B; her actual well-fitting size is likely 30C or 30D. The Bratabase and Reddit's r/ABraThatFits communities popularised the "BOUX" or modern fitting method that uses the actual underbust measurement as the band size, and this shift has moved many women out of conventional 32-38 A-D sizing into what the industry misleadingly calls "extended" sizes.
Sister sizing — the fact that 34C, 32D, and 36B all contain the same cup volume with different band lengths — is poorly understood and further complicates self-measurement. A cup size is not an absolute volume; it is a ratio between bust and underbust measurements. This means cup size is meaningless without band size, and a D cup on a 32 band is dramatically smaller in absolute volume than a D cup on a 38 band. The calculator on this page uses the modern fitting method rather than the traditional +4 approach, which is why results may differ from traditional department store sizing.
Average bra size by country
Bra sizing conventions vary by country, making direct comparison difficult. The US uses the inch-based system (32A, 34B, 36DD, etc.). The UK uses similar but not identical conventions. Europe uses centimetre-based EU sizing (75B, 80C, 85D). France uses its own French system. Japan uses its own sizing starting from A-65. Converting between systems requires care: a UK 34D and a US 34D are the same; a French 90C and a US 38C are different despite the letter similarity.
When converted to US equivalent sizing, cross-national averages include: USA 34DD, UK 36DD, Australia 14D (approximately US 36D), Canada 36C, Norway C-D (largest average in Europe), Japan A-65 (approximately US 30A), France 85B (approximately US 32B), Germany 80C (approximately US 36C), Brazil 40B, India 34B. These figures come from Worlddata.info's aggregated dataset combining national health surveys, retail sales data, and industry fitting studies, with varying precision by country — national figures from countries with large retail datasets (US, UK, Germany) are more reliable than estimates for countries with limited data. The common thread: average cup size in all countries has increased over the past 30 years, tracking BMI trends globally.
Frequently asked questions
Use a soft fabric measuring tape. For underbust: measure snugly around your ribcage directly under your breasts, keeping the tape parallel to the floor. Breathe out and measure relaxed. For the bust: measure around the fullest part of your breasts, keeping the tape level and not pulling tight. Lean slightly forward to let the breast tissue hang naturally. Take measurements on bare skin or a single thin unpadded bra. Repeat twice to confirm consistency.
Bra sizing is not standardized across manufacturers. A 34C in one brand may fit like a 34B or 34D in another because each brand uses different block patterns and grading rules. This is sometimes called "vanity sizing" in the industry, though it is more accurately a result of different manufacturing traditions. Your measured size is a starting point: always try bras on before purchasing if possible, and be aware that the same nominal size will fit differently across brands.
National average bra sizes vary substantially. The United States and United Kingdom both average approximately DD/E cups in recent surveys, having shifted significantly from the 34B average commonly cited from the 1990s. This shift reflects changes in average body size and improved measurement practices. Norway and some Scandinavian countries also report larger average sizes. Japanese averages are considerably smaller (around A-cup in national surveys), reflecting differences in body frame distribution. French and German average sizes tend toward B-C cups in population data.
Breast size typically increases during pregnancy and lactation. Underbust circumference often increases during pregnancy due to rib cage expansion. After weaning, breast volume may change again. It is recommended to measure and re-evaluate bra size at each stage: early pregnancy, late pregnancy, immediately postpartum, during active nursing, and after weaning. Sizes can shift by two or more cup letters and one to two band sizes across these stages.
A correctly fitted bra has: a band that lies horizontally and snugly around the ribcage without riding up; cups that fully contain the breast tissue without spillage, gaps, or puckering; straps that provide light support but are not carrying the majority of the load (the band should bear 80-90% of the weight); a central gore (the part between the cups) that lies flat against the sternum. If the band rides up at the back, the band is too large. If there is spillage or the underwire sits on breast tissue rather than chest wall, the cup is too small.
In the United States, 34DD is currently the most commonly sold bra size according to retail sales data, followed by 36DD and 34D. This represents a dramatic shift from 1992 when 34B was the most common size. In the UK, 36DD is the most common. The "most common" size is derived from retail sales, which skews toward sizes that major retailers carry in depth — sizes above 38 and above E cup are often sold as "extended sizes" with fewer retailers, which means the retail data likely underrepresents women in those sizes. When professional fitting studies are used rather than retail sales, the distribution shifts: more women fit into larger cups with smaller bands (30-34 bands in D-G cups) than retail data suggests, because these sizes are less stocked and women are often wearing a sister size that is available rather than the size that actually fits.
The most common signs of an incorrectly fitting bra are: band riding up at the back (means band is too loose — go down a band size and up a cup size); underwire sitting on breast tissue rather than fully below it (means cup is too small); cups gaping or wrinkling (means cup is too large or shape is wrong); straps falling off or digging in significantly (straps should provide minimal support — the band should do most of the work); quadruple boob effect where tissue spills over the top of the cup (cup too small). The single most important fit point is the band: it should sit horizontal across your back, parallel to the floor, and should feel snug enough that you cannot pull it more than two fingers away from your body. Most women wearing a 36B who have fitting issues would benefit from trying a 34C (the sister size) — same cup volume, snugger band.