How do I measure my bra size at home?

Take two measurements with a soft tape. First, measure your underbust: wrap the tape snugly around your ribcage directly beneath the bust, keeping it level, and round to the nearest even number for your band size. Second, measure your bust at its fullest point with the tape level and loose. Subtract the underbust from the bust, and each inch of difference is one cup size, so a four-inch difference is a D cup. Measure in a non-padded bra or without one for the most accurate result.

What is the difference between band size and cup size?

The band size is the number, like 34, and it comes from your underbust measurement around the ribcage. The cup size is the letter, like C, and it comes from the difference between your bust and underbust measurements. The two are linked: the band supports most of the bra's weight and the cup holds the breast tissue. A common mistake is treating them separately, but changing one changes how the other fits, which is why the cup letter only makes sense paired with a specific band.

Why do different bra size calculators give different results?

Because they use different band-measurement methods. The modern snug method takes the band directly from your underbust measurement, while the older plus-four method adds four inches to it. Adding four inches inflates the band number and, because the cup is measured against that larger band, shrinks the cup letter. The same person can come out as 32DD on the snug method and 36B on the plus-four method. This calculator shows both side by side so you can see exactly where the disagreement comes from.

Should I add four or five inches to my band measurement?

No, not with modern bras. The plus-four rule dates from an era of less stretchy fabrics and is the single biggest cause of poor bra fit today. Adding inches gives you a band that is too loose, which then rides up at the back and forces the breast weight onto the shoulders. It also makes the calculated cup too small. Fitting communities and most current retailer guides recommend measuring the band snugly and using that number directly, which is the default this calculator uses.

What are sister sizes and when should I use them?

Sister sizes are different band-and-cup combinations that share the same cup volume. To find them, move the band up one size and the cup down one letter, or the band down one size and the cup up one letter. For example, 34C, 32D, and 36B are all sisters. They are useful when the cup fits but the band feels too tight or too loose: instead of changing both dimensions, you swap to a sister size to adjust only the band while keeping the same cup volume. This is especially handy across brands that run tight or loose.

How do US, UK, and EU cup sizes differ?

Up to a D cup the letters are mostly the same, but they diverge above D. The US system uses doubled letters such as DD, DDD, and then G, H. The UK system goes DD, E, F, FF, G. The EU and French systems use single continuous letters, so a US DD is roughly an EU E. Band numbers differ too: a US or UK 34 band is an EU 75 and a French 90. This is why a bra bought abroad can be labelled completely differently even though it is the same physical size, and why the conversion panel is useful.

My band rides up at the back, what size should I try?

A band that rides up is almost always too big, not too small. The band should sit firm and level around your ribcage, parallel to the floor, and do most of the supporting work. If it climbs up your back, go down a band size and up a cup letter to keep the same cup volume, for example from 36C to 34D. Counterintuitively, people with a riding-up band have usually been wearing a band that is several sizes too loose, which is exactly what the old plus-four method produces.

How do I know if my cup size is wrong?

Look for two telltale signs. If the cups wrinkle, gape, or have empty space at the top, the cup is too big, so go down a letter. If breast tissue spills over the top or sides, creating a double-bust line or bulging near the underarm, the cup is too small, so go up a letter. Quad-boob, where the cup cuts across the breast, is the classic sign of a cup that is too small, often paired with a band that is too loose. Adjust the cup first while keeping the band snug.

How often should I remeasure my bra size?

About every six months, and any time your weight changes noticeably, after pregnancy or breastfeeding, or when starting hormonal birth control or menopause. Bra size is not fixed: ribcage and breast tissue both change over time, and even a one-inch shift in measurement can move you a full cup size. Many people wear the same size for years out of habit even after their body has changed, which is one reason an estimated majority of women are thought to wear the wrong size. A quick remeasure prevents that.

Can I measure my bra size without a properly fitting bra?

Yes. The most accurate measurements are taken either braless or in a thin, non-padded, non-push-up bra, because a padded or ill-fitting bra distorts the breast shape and inflates the bust figure. If you only have a poorly fitting bra, measuring without one is better. Stand upright, keep the tape level and snug for the band and level and loose for the bust, and avoid pulling the tape tight across the fullest point, which would understate your cup size.

Bra Size Calculator – Band, Cup & Sister Sizes

Bra Size Calculator

The Bra Size Calculator works out your band and cup size from an underbust and a bust measurement in inches or centimetres. It returns your US size plus UK, EU, French, and Australian conversions, your sister sizes for brand-to-brand fit adjustments, and a side-by-side comparison of the modern snug method against the legacy plus-four method that explains why bra calculators so often disagree. Use it as the size to try first, then refine with the sister sizes.

What Is the Bra Size Calculator?

The Bra Size Calculator turns two simple body measurements, your underbust and your bust, into a complete bra size with the band number and cup letter worked out for you. Enter the figures in inches or centimetres and it returns your recommended US size along with UK, EU, French, and Australian conversions, your sister sizes, and a side-by-side comparison of the two measuring methods that cause so much confusion. Studies have long suggested that a large majority of people wear the wrong bra size, and as the standard reference on bra sizing explains, the usual culprit is a band that is too loose paired with a cup that is too small.

What sets this calculator apart is that it does not hide the method behind a single number. Given that the modern snug approach and the older plus-four approach can produce sizes two or three cup letters apart from the very same measurements, the tool shows both, so you can finally see why one shop measured you as a 36B and an online calculator insists you are a 32DD. On top of that, it works out your sister sizes for brand-to-brand fit adjustments and converts your size into five regional labelling systems, which takes the guesswork out of buying lingerie from international retailers.

How Band and Cup Size Are Calculated

A bra size has two parts that come from two different measurements. The band, the number such as 32 or 34, comes from your underbust measurement taken snugly around the ribcage and rounded to the nearest even inch. The cup, the letter such as B or D, comes from the difference between your bust and your underbust, where each inch of difference works out to one cup size: a one-inch difference is an A, two inches a B, three a C, four a D, and so on up through DD and beyond. So someone measuring 32 inches under the bust and 36 across it has a four-inch difference and lands at a 32D.

The two parts are not independent, which is the point most people miss. Because the cup is measured relative to the band, getting the band wrong automatically corrupts the cup letter. That said, the arithmetic itself is straightforward, and once you have measured carefully the calculator does the rounding and lettering instantly. If you are sizing other garments at the same time, the same careful measuring habit pays off with our jeans size calculator and our shoe size calculator, both of which turn body measurements into international sizes.

Snug Versus Plus-Four: Why Calculators Disagree

The single greatest source of bra-size confusion is the existence of two competing methods for setting the band. The modern snug method, recommended by dedicated fitting communities and increasingly by retailers, takes the band straight from the underbust measurement. The legacy plus-four method, a holdover from the mid-twentieth century when fabrics had little stretch, adds four inches (or five, rounded to an even number) before setting the band. The fit guides published by major retailers increasingly favour the snug approach precisely because the added inches throw the rest of the calculation off.

The consequences ripple through the whole size. Adding four inches turns a snug 32 band into a loose 36, and because the cup is then measured against that inflated band, a genuine DDD can collapse into a B. The result is the textbook bad fit: a band that rides up the back because it is too loose, and cups that overflow because they are too small. The calculator displays both methods together so the gap is visible, and it defaults to the snug result. With that in mind, if a size ever looks wildly different from what you expected, the method is almost always the reason.

Sister Sizes and International Conversions

Once you know your size, two further tools help you actually buy bras that fit. Sister sizes are different combinations that hold the same cup volume: move up a band and down a cup letter, or down a band and up a cup letter, and the cup stays the same size while the band changes. The table below shows how a 34D relates to its neighbours, which is invaluable when a brand runs tight or loose and you need to tweak only the band.

Adjustment	Size	Band Fit	Cup Volume
Sister size down	32DD	Tighter band	Same as 34D
Your size	34D	Standard	Reference
Sister size up	36C	Looser band	Same as 34D

International conversions matter just as much, because the same physical bra carries different labels around the world. A US 34D is a UK 34D, an EU 75D, a French 90D, and an Australian 14D, and above a D cup the letters diverge further still. The ISO clothing size standards underpin the body-measurement logic, while each region layers its own labelling on top. The calculator handles all five systems at once, so ordering from an overseas label becomes a matter of reading the panel rather than hunting through a conversion chart.

Accuracy and Limitations

The calculator applies the standard band-and-cup arithmetic exactly, so for clean measurements the size it returns is mathematically correct. It rounds the band to the nearest even inch and the cup to the nearest whole inch of difference, matching how sizes are actually labelled. The international conversions follow the widely used band relationships, EU bands in centimetres, French bands fifteen above the EU number, and Australian bands twenty below the US number.

Even so, a calculated size is a starting point, not a guarantee. Bra fit depends on breast shape, projection, root width, and the specific cut and brand of the bra, none of which a measurement captures, and brands vary enough that two bras of the same printed size can fit very differently. Soft tissue also compresses differently depending on how firmly you hold the tape, so small measuring inconsistencies can shift the result by a cup. Treat the output as the size to try first, then use the sister sizes and a fitting room or a brand's return policy to refine from there.

The Most Common Bra Sizing Mistake: Sizing Up the Band Instead of the Cup

In my experience the error that traps the most people is responding to overflowing cups by buying a bigger overall size, which usually means a bigger band too. That is backwards. When the cups overflow but the band already feels loose, the fix is to keep the band the same or go down a band size and instead go up a cup letter, because the band should stay firm and do the supporting work. Going up a full size in both dimensions just loosens an already-loose band further and barely improves the cup. With that in mind, always diagnose the band and the cup separately: get the band snug first, then adjust the cup, and reach for a sister size when only the band needs changing. That single habit corrects the majority of long-standing fit problems.

Bra Size Calculator