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Our sizing method and review process

How Our Bra Size Calculator Works

This page explains exactly what our tools do, where the rules come from, what they cannot tell you, and how we check and correct our guidance. It describes the code as it currently runs.

Last reviewed:

What does the calculator measure and calculate?

You enter two measurements: your underbust and your bust, in inches or centimeters. The calculator follows the tape method published by two UK retailers, Brastop and Boux Avenue, which agree on every rule they both state. It works in inches because US and UK bands are inch-based, and then shows the result in your chosen sizing system. We checked both methods on 17 October 2026.

  1. 1 Convert unitsCentimeters are converted to inches using 1 inch = 2.54 cm. No UK brand we checked publishes a centimeter-to-band table, so this conversion is our own implementation choice.
  2. 2 Work out the bandRound the underbust up to the next whole inch. If that number is odd, round up again to the next even number. That is the band.
  3. 3 Round the bustRound the bust to the nearest whole inch. Exactly half an inch rounds up.
  4. 4 Find the differenceSubtract the band from the rounded bust. Boux Avenue’s worked example confirms the rounded band is used: 39 − 34 = 5, giving DD.
  5. 5 Find the cupEach inch of difference is one UK cup step, using Brastop’s published table from 1 inch (A) to 15 inches (K).
  6. 6 Show it in your systemThe UK result is converted to US, EU, French or Australian labels using brand-published charts, and shown as a suggested starting size with nearby sister sizes.

Worked example

Underbust 31.5 in → rounded up to 32 (already even) → band 32. Bust 36.5 in → rounds to 37. Difference 37 − 32 = 5 in → DD. Suggested starting size: 32DD.

Do we add 4 inches to the band?

No. Some older and some US methods add 4 or 5 inches to the underbust. The UK method we follow says not to add anything (“do not add anything extra on top!”, Brastop). A UK study (White and Scurr, 2012) found that the traditional fitting method it tested overestimated band size and underestimated cup size; the study does not say exactly which tape method that was.

How does this compare with US size charts?

Some US brands publish their own measuring method, and some add inches to the underbust. For example, a Hanes size chart hosted by Target says to add 5 inches to an odd underbust measurement and 4 inches to an even one. Our calculator does not add inches, so it can suggest a smaller band and a larger cup letter than a chart like that.

We calculate on the UK cup sequence and then show the result in US, EU, French or Australian labels using brand-published conversion charts. US brands do not all label large cups the same way (for example DDD or F), so if a brand publishes its own chart, check it for the item you are buying.

How do we round measurements, and what are the limits?

Rounding rules, supported ranges and boundary handling
SituationWhat the calculator does
Underbust 30.0 inBand 30 (already whole and even)
Underbust 30.1 inRounds up to 31, which is odd → band 32
Underbust 29 inOdd → band 30
Bust 36.5 inRounds to 37 (halves round up); 36.49 rounds to 36
Centimeters on an inch boundarySmall floating-point errors are removed, so 76.2 cm is exactly 30 in
Supported bands28 to 48. Outside this, we say the size is outside our range instead of guessing.
Difference of 1 to 15 inA to K, from the published table
Difference of 16 or 17 inKK and L, which follow the UK cup sequence but are not in the published table. These results are labeled as extrapolated.
Difference of 0 or less, or more than 17 inNo size is suggested. We explain why and point to measuring help and specialist fitting.
Implausible inputValues outside 20–70 in (or the cm equivalent), or a bust smaller than the underbust, are rejected with a message, which also catches many unit mix-ups.

Which cup sequence do we use?

UK sizing, confirmed across several UK brand and retailer charts: AA, A, B, C, D, DD, E, F, FF, G, GG, H, HH, J, JJ, K, KK, L. There is no I cup in UK sizing, and each double letter is a full cup step. We do not support cups beyond L because we did not find a UK brand chart that lists them.

Why is the result a starting size, not a guaranteed fit?

  • Two measurements cannot describe breast shape. Brastop notes that two people with the same measurements may need different sizes.
  • Some specialist retailers, such as Bravissimo, say they do not use tape measures at all and fit by eye.
  • In an Australian study of 104 women (McGhee and Steele, 2010), sizes from measurement systems differed significantly from the correct size, and 85% of participants were wearing ill-fitting bras.
  • Brands and styles differ in cup shape, wire width and band firmness.

For these reasons every result says “suggested starting size”, and we point you to the fit checker and sister sizes.

How do our other tools work?

Sister sizes

One band size down and one cup step up, or one band up and one cup down, through the UK sequence (so 32DD gives 30E and 34D). If the next size falls outside 28–48 or past L, we say so instead of inventing a label.

Size converter

Converts an existing size label between UK, US, EU, French and AU/NZ sizing using brand-published charts, not by converting inches to centimeters. Bands are consistent across charts. US labels have known aliases (for example DDD or F). From UK G upwards, EU charts disagree, so we show both. Where no consistent published mapping exists, we say it is unsupported.

Size chart

Shows UK size labels. Selecting a label is not a recommendation.

Fit checker

A fixed set of questions and rules written in advance. It does not use AI to generate advice, does not diagnose medical conditions, and does not assign a new size from one symptom.

Style and brand guides

Style descriptions are general. Brand guides summarize each brand’s own published information with the date checked; we do not publish “runs small” claims or test results.

How do we handle product information?

We have not yet published product listings. When we do, each listing will record the product page, the size label to select, the price and when it was checked, and evidence for whether your exact size and color was listed as available. A size being in a product’s range is shown separately from availability. We never swap in a sister size automatically, and affiliate commission is not used to choose or order results.

What happens to my measurements?

Calculations run in your browser. Measurements and fit-checker answers are not sent to us, saved, or added to the web address. Size labels passed between our tools use the part of the address after “#”, which browsers do not send to servers. See our privacy notice.

Who builds and checks these tools?

Current review status: our method and guides have not yet been reviewed by an independent professional bra fitter. They are based on the published sources listed below. We plan to have the method and core guides reviewed by an experienced fitter, and we will name the reviewer and the review date here when that happens.

The calculations are covered by automated tests that check them against the published rules and worked examples, including rounding boundaries, the UK double-letter sequence, sister sizes and conversion tables.

How can I report a mistake?

If an instruction, result, conversion or brand detail looks wrong, please tell us through the contact page. Include the page and, if relevant, the size label involved. Please don’t send your body measurements; we don’t need them to check a rule. We review reports against the sources and update this page and the “last reviewed” date when we change something.

Frequently asked questions

Is a bra size calculator accurate?

It is a reasonable starting point, not a precise answer. Brands that use tape methods describe the result as a starting point, and research has found that tape-based sizing often differs from a professionally fitted size. Breast shape, the style of bra and brand differences all matter.

Do I add 4 inches to my band size?

Not with our method. We round the underbust up to a whole inch and then up to an even number, and add nothing else. The two UK retailers whose method we follow both say not to add inches.

Why does my result differ from a shop fitting?

Fitters judge fit by eye with real bras, which can account for shape and style in a way two tape measurements cannot. Some specialist retailers do not use tape measures at all. If a fitting and the calculator disagree, the bra that fits you well is the better guide.

Sources and review status

Sources last checked: . This page has not yet been reviewed by an independent professional bra fitter.

Spotted something that looks wrong? Report a correction or read our sizing method.

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