How the Bra Size Converter Works and Why Sizes Differ
A bra size converter translates a band-and-cup size between the different systems used around the world and, just as usefully, works out a starting size from two body measurements. A bra size has two parts: the band number describes the ribcage under the bust, and the cup letter describes how much larger the bust is than that band. The Helperzy version maps the US, UK, EU, French, Italian, Australian, and Japanese systems, because the same body can be labelled 34C in the US and UK, 75C in the EU, 90C in France, and 3C in Italy. Anyone shopping across borders, reading a foreign brand's chart, or trying to decode an unfamiliar label runs into this straight away.
The conversion has two independent halves. The band converts mostly by a fixed offset: US and UK bands share the same numbers in inches, the EU and Japanese bands use centimetres so a 34 becomes roughly 75, the French band adds fifteen to the EU number so it becomes 90, and the Italian band uses small numbers like 3. The cup letter is trickier, because although US and UK cups line up for the first few letters, they diverge for larger cups where the UK inserts extra letters such as DD and FF. This tool lays the systems side by side so you can read straight across rather than guessing, and it keeps the band and cup steps separate so an error in one does not corrupt the other.
Just as important is the measurement path, which finds a size from your body rather than from another label. Measure your underbust snugly in inches to get the band, rounding to the nearest even number, and measure around the fullest part of the bust. Subtract the band from the bust measurement, and each inch of difference is one cup: a one-inch difference is an A cup, two inches a B, three inches a C, four inches a D, and so on up the alphabet. For example, a 32-inch underbust with a 35-inch bust gives a three-inch difference, which is a 32C. This method is the reliable route because it depends on your actual shape rather than on how one brand chooses to label a garment.
The standout feature here is sister sizes, which solve a very common fitting problem. Cup volume depends on both the band and the cup together, so if a band feels too tight or too loose you can move to the next band and shift the cup by one letter to keep the same volume. That is why a 30D, a 32C, and a 34B all hold a similar cup, and why a 34DD is a sister size to a 36D and a 32E. The converter shows these sister sizes explicitly, which is invaluable when your exact size is sold out, when a particular band digs in, or when a brand simply does not stock your combination. Seeing the alternatives gives you several correct options rather than a single dead end.
A few honest cautions matter. Brands vary a great deal, so two garments both labelled 34C can fit noticeably differently depending on the maker, the style, and the fabric stretch, which means any converter gives a strong starting point rather than a guarantee. Sizes also shift with the menstrual cycle, weight changes, and pregnancy, so it is worth re-measuring occasionally. Treat the sister-size list as a fitting toolkit: if the recommended size is not quite right, try one band up or down with the cup adjusted before assuming the whole size is wrong. Everything here runs locally in your browser; your measurements and sizes are never uploaded, logged, or stored, so this personal information stays entirely private on your own device.
