Wrap the tape snugly around the ribcage, directly under the bust, keeping it horizontal and flat against the skin without pulling it tight.
Wrap the tape around the fullest part of the bust, keeping it level across the back, without a bra or with a well-fitting unpadded one.
| System | Size |
|---|
Why a UK DD is not the same as a US DD
British high street chains such as Marks & Spencer, Bravissimo, or John Lewis use a scale where certain letters double up, so after D the next step is DD, then E, then F, then FF, rather than moving straight to the next single letter. American size charts skip that doubling and move from D to E directly, so a UK DD actually lines up closer to a US E, and a UK size bought without checking this ends up with too little cup volume when read as if it were a US label. This mismatch is one of the most common ordering mistakes when buying from an American retailer that ships to the UK, or the other way round.
Band and cup, calculated from two separate measurements
The table above works out the band from the underbust figure and the cup from the difference between the bust and underbust figures, then applies each country's own letter scale to that same cup step. A band that is too loose is the single most frequent fitting problem reported by UK bra fitters, more often than a cup that is the wrong letter, which is why getting the underbust measurement right matters as much as the bust figure.
Measuring without guessing at a shop mirror
Take both measurements over bare skin or a thin, unpadded bra, standing straight with the arms relaxed, and read the tape at the end of a normal breath rather than while holding it in. A measurement taken over a padded or ill-fitting bra, or with the tape pulled tight to get a smaller number, is the reason many self-measurements do not match the size a professional fitting later confirms.
Limits of this converter
The figures follow a standard calculation method rather than any single retailer's own size chart, and UK brands in particular vary a fair amount in cut between an underwired everyday style and a soft, unstructured one. Treat the result as a solid starting point for trying styles on, not as a fixed number, since sizing still differs from one label to the next.