A bra size calculator turns two tape-measure readings into a size label. That is all it can honestly do, and it is worth being clear about that up front, because a bra size is not a measurement of a body — it is a label describing a garment that is meant to fit that body. The arithmetic below gives you a well-founded starting point. Whether a particular bra in that size actually fits depends on the manufacturer, the style and the cut, and no calculator can predict that.
What this page adds beyond the arithmetic is the conversion between systems, which is the reason most people search for this in the first place. The United States, the United Kingdom, continental Europe and Australia all label the same garment differently, and the differences are not consistent from one part of the size range to another. Arb Digital publishes a free tools library covering practical everyday measurement problems, and this is a straightforward one that is made unnecessarily confusing by four incompatible labelling conventions.
What This Bra Size Calculator Does
It takes an underbust measurement and a bust measurement, in inches or centimetres, and derives two things: a band size from the underbust, and a cup letter from the difference between bust and underbust. It then writes that same size in US, UK, EU and Australian notation, and shows the cup difference in inches so you can see how the letter was reached.
The sub-line under the headline result gives the two sister sizes — one band down with one cup up, and one band up with one cup down. Those describe roughly the same cup volume with a different band circumference, and they are the first thing to try when a size is close but not right.
The method selector matters more than it looks. The traditional approach adds four inches to an even underbust measurement, or five to an odd one, before setting the band. That convention comes from a period when band fabric had very little stretch and needed the extra circumference. Modern elastics do not, and most current fitting guidance takes the band straight from the underbust measurement. The two methods can put you two band sizes apart, which is a large difference, so the selector lets you see both rather than pretending one is universally correct.
How to Use It
- Choose your units. Inches or centimetres — the calculation converts internally, so use whichever your tape measure shows.
- Measure the underbust. Stand upright, run the tape around the ribcage directly under the bust, and pull it snug but not tight. Keep it level all the way around; a tape that rides up at the back adds an inch of error.
- Measure the bust. Around the fullest point, with the tape resting rather than compressing. Standing straight matters here; leaning forward changes the reading substantially.
- Pick a band method. Start with the modern method. Switch to traditional if you are comparing against an older size chart or a brand that still uses it.
- Read the four systems. The letters and the numbers both change between systems, so copy the whole label, not just the letter, when ordering from another country.
The Formula / How It's Calculated
The band comes from the underbust. Under the modern method the underbust measurement in inches is rounded to the nearest whole number and then rounded up to the next even number if it is odd, because bands are made in even sizes. Under the traditional method, four inches are added to an even underbust figure and five to an odd one before that rounding.
The cup comes from the difference. Cup difference = bust − band, in inches. Each full inch of difference is one cup letter: zero inches is AA, one inch is A, two is B, three is C, four is D, and so on up the alphabet with the specific letters depending on which country's system you are reading.
Worked example, matching the values the page loads with. An underbust of 34 inches is already even, so under the modern method the band is 34. A bust of 37 inches gives a difference of 37 − 34 = 3 inches, which is a C cup. The result is 34C in both US and UK notation. In EU notation the band converts to 75, giving 75C. In Australian notation the band converts to 12, giving 12C. The sister sizes are 32D — one band smaller, one cup larger — and 36B.
In centimetres the same logic runs after dividing by 2.54. An underbust of 86.4 cm is 34.0 inches and produces the same answer. Note that EU and UK sizing charts often quote underbust ranges in centimetres directly rather than converting, which is one source of the small disagreements between charts discussed below.
Why UK, US, EU and Australian Sizes Are Different
The band numbers differ because each system measures a different thing. US and UK bands are the underbust circumference in inches. EU bands are approximately the underbust circumference in centimetres, so a US 34 corresponds to an EU 75 rather than to 86. French, Spanish and Belgian labels add a further 15 to the EU number, which is why a French 90 and an Italian 3 and a US 34 can all describe the same band. Australian and New Zealand bands use the same numbering as their dress sizes, so a US 34 is an AU 12.
The cup letters diverge higher up the range, and this is where cross-border ordering goes wrong. US, UK and EU systems agree on A, B, C and D. At five inches of difference all three still say DD, though the US often writes it as E. At six inches the US writes DDD or F, the UK writes E, and continental Europe writes F. At seven inches the US writes G, the UK writes F, and the EU writes G. The UK system then inserts doubled letters — FF, GG, HH — which the US system does not use at all.
The consequence is simple and important: a UK F cup and a US F cup are not the same cup. Above a D, always convert rather than assuming the letter transfers. The calculator does that conversion for you, and the underlying body measurement is the only thing that stays constant across all four labels.
The measurement procedures themselves are standardised even where the labels are not. ISO 8559-1:2017, Size designation of clothes — Part 1: Anthropometric definitions for body measurement defines how bust and underbust girth are taken, and the European series BS EN 13402 Size designation of clothes sets out how those body dimensions map onto labels in centimetres. Both are about the body measurement, not about any one manufacturer's pattern.
Sister Sizes and Why the Cup Letter Alone Means Nothing
A cup letter is not a fixed volume. It describes the difference between bust and band, so the same letter on a different band is a different cup. A 30D, a 34D and a 38D have visibly different cup volumes, because each is four inches larger than a band that is itself four inches apart from the next.
Sister sizes follow from this. Moving up one band size and down one cup letter — 34C to 36B — keeps the cup volume roughly constant while loosening the band. Moving down one band and up one cup — 34C to 32D — keeps the volume and tightens the band. These are the two sizes to try when a bra is nearly right, and they are shown in the sub-line of the result for exactly that reason.
It also explains a common frustration. Someone told they are a D cup at one band size, who then loses or gains weight around the ribcage, may find their cup letter changes even though the bust itself has not. The letter is a difference, and differences move when either number moves. The percentage change calculator is useful if you are tracking measurement changes over time, and the waist to hip ratio calculator and body surface area calculator cover other body-measurement ratios.
Where Calculated Sizes Stop Being Reliable
Three things routinely make the calculated size the wrong size to buy.
Brand variation. There is no enforced standard for what a 34C bra must physically measure. Two manufacturers working from different fit models will cut noticeably different garments for the same label, and the same brand often varies between its own ranges. A size that fits in one line is a starting point in another, not a guarantee.
Style. Cup shape changes fit as much as cup size. A balconette, a plunge, a full-cup and a moulded T-shirt style with the same label will sit differently on the same body, because they distribute the same volume across a different shape. This is why a calculated size can feel too small in one style and too large in another.
Shape rather than volume. The bust-minus-band arithmetic captures volume and nothing else. Two people with identical measurements can need different cup shapes depending on how that volume is distributed, and no two-number calculation expresses that.
Treat the calculated size as the first size to try, expect to try its two sister sizes as well, and let how the garment sits decide. A band that rides up at the back is too loose; a cup that gaps or spills is the wrong volume or the wrong shape.
Measuring Accurately: The Errors That Move a Full Size
The calculation is only as good as the two numbers you feed it, and both are easy to take badly. A tape that is not level around the back is the most common error and typically adds half an inch to an inch to the underbust — enough to move the band a full size. Measure in front of a mirror, or have someone check the back.
Tape tension is the second. The underbust should be snug enough that the tape does not slide, and the bust should be loose enough that the tape does not compress. Applying the same tension to both readings reduces the difference and pushes the cup letter down, which is why some people calculate a size that is obviously too small.
Posture is the third. Leaning forward, standing very upright, or holding a breath all change the readings. Stand normally, breathe out gently, and take each measurement twice — if the two differ by more than a quarter of an inch, take a third. For unit conversions, the length converter and the clothing size converter handle inches, centimetres and general garment sizing.
Accurate size guidance is one of the strongest levers on return rates. Arb Digital builds interactive tools and product content that help shoppers pick correctly the first time.
Browse Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Assuming the cup letter transfers between countries — a UK F and a US F are different cups. Above D, always convert the whole label.
- Letting the tape ride up at the back — an unlevel tape adds most of an inch to the underbust and can move the band a full size.
- Pulling the bust measurement tight — compressing the tape shrinks the difference and produces a cup letter that is too small.
- Mixing the two band methods — adding four inches and then using a modern chart puts you two band sizes out. Pick one method and stay with it.
- Treating the result as a fixed size across brands — there is no enforced standard for what a given label must physically measure, so the same size varies between makers and between styles.
Related Free Tools From Arb Digital
Use the clothing size converter for general garment sizing across countries, the shoe size converter for footwear, the ring size converter for jewellery, and the length converter to move between inches and centimetres. The waist to hip ratio calculator covers another common body-measurement ratio. Everything else is in the free online tools hub.
Frequently Asked Questions
Only if you are working from a chart that expects it. The plus-four method comes from an era of low-stretch band fabric; most current fitting guidance takes the band straight from the underbust measurement. The two approaches can differ by two band sizes, so the method selector lets you compare them.
At D itself, yes — US, UK and EU all agree on A through D. They diverge above that. At six inches of difference the US writes DDD or F, the UK writes E, and continental Europe writes F, so above a D the letter alone is not enough information.
EU band numbers approximate the underbust in centimetres rather than inches, so US 32 is EU 70, US 34 is EU 75, US 36 is EU 80 and so on in steps of five. French and Belgian labels add a further 15 to the EU number.
Two labels describing roughly the same cup volume on different bands. Going up one band and down one cup, such as 34C to 36B, keeps the volume and loosens the band; going down one band and up one cup does the reverse. They are the first alternatives to try when a size is close but not right.
Because a label is not a standardised physical measurement. Manufacturers cut from different fit models, and cup shape varies between balconette, plunge, full-cup and moulded styles. Use the calculated size as a starting point and try its two sister sizes alongside it.
Yes. Set the unit selector to centimetres and enter both readings that way. The calculation converts internally at 2.54 centimetres per inch, because the cup letter is defined by a difference in inches regardless of the tape you used.
It can, because both numbers can move independently. A change in ribcage measurement alters the band, and because the cup letter is the difference between bust and band, the letter can change even when the bust measurement has not. Re-measuring periodically is more reliable than assuming a fixed size.
This tool performs measurement arithmetic and size-system conversion only. It is not medical or health guidance, and it cannot account for differences between manufacturers, styles or body shape.
