How the calculator works

The calculator rounds your snug underbust to the nearest even inch for the band, then counts one cup letter for each inch your bust measures above that band. It converts the result into each country's labels using published tables.

The formula

1. Band: your snug underbust, rounded to the nearest even inch. Exact odd numbers round up.
2. Bust figure: your standing bust. In detailed mode, it is the average of your standing, leaning and lying readings.
3. Cup: bust minus band, rounded to the nearest inch. 0 is AA, 1 is A, 2 is B, and so on through the letters.
4. Correction: if your bust comes out smaller than the rounded band, the band moves down one size so the result stays usable.

For example, a 33¼ inch underbust rounds to 34. A 38 inch bust is 4 inches above that, so the result is 34D. Measurements in centimetres are divided by 2.54 first.

Why it suggests more than one size

Rounding hides how close you are to the next size. So the calculator also shows a neighbouring size when:

• your bust is within 0.2 inches of the line between two cups,
• your underbust is 0.6 inches or more from the band it rounded to,
• your tight underbust is at least 2 inches below the band, or your loose underbust at least 3 inches above it.

These thresholds are this site's own judgement calls, chosen to flag close calls. They have not been tested in a study.

Why it does not add 4 inches to the band

An older method adds 4 or 5 inches to the underbust to get the band. Critics of that approach, sometimes called the “war on plus four”, argue it fails to fit most women. This calculator uses your underbust as measured.

Research points the same way. White and Scurr (2012), writing in Ergonomics, compared the traditional tape method with professional fitting criteria in 45 women wearing an underwired bra. The tape method overstated band size and understated cup size, and the gap grew with larger bands. The authors recommended that women learn to assess fit for themselves, with less emphasis on absolute size. That is why every result here is called a starting size, and why the site includes a fit checker.

Why European charts can give a different size

This trips up many shoppers. There are two ways to reach a European size, and they do not always agree:

• Converting the label. A UK or US 30 is sold as an EU 65. This is what size conversion charts show, and what this calculator does.
• Measuring directly. The European standard, EN 13402, uses your actual underbust in 5 cm steps, with cups in 2 cm steps.

Take a 76 cm underbust and a 92 cm bust. The calculator gives 30E in the UK, which converts to 65F in the EU. A chart that measures directly gives 75C. When you shop in a European system and the two differ, the result shows both, so you can try the direct size if the first band feels too tight.

Where the tables come from

Band and cup tables follow the published summary of national sizing systems, which covers EN 13402 for Europe and JIS L4006 for Japan. Conversions to Japanese and Italian sizes are approximate, because their cups are defined differently and brands vary. US cups above DDD also differ between brands.

Band labels and cup letters are checked by automated tests against these published values before each update goes live.

What it cannot do

• It cannot see the shape of your breasts, which often decides whether a particular style fits.
• It cannot account for how each brand cuts its bras. Check the brand's own chart when there is one.
• It has not been tested against professional fittings. It applies a widely used method, carefully and openly.
• It is not medical advice. If you have pain, or notice a change in your breasts, speak to a doctor.

Sources

• White, J. and Scurr, J. (2012). Evaluation of professional bra fitting criteria for bra selection and fitting in the UK. Ergonomics, 55(6), 704 to 711.
• Bra size, Wikipedia, for national sizing tables and the history of the add-4 method.