Modern band-rounding method, with US / UK / EU side-by-side.
The classic +4/+5 band formula (where you added several inches to your underbust measurement) was retired by professional fitters in the 1990s — it consistently produced bands that were too loose and cups that were too small. The modern method is simpler: round your band measurement to the nearest even inch, then derive the cup from the inch-by-inch difference between band and bust. The widget below does exactly that and then translates the US size into UK and EU labels.
Sizing varies meaningfully between brands. Treat this as a starting point and try at least one size up and down in band/cup combinations for the best fit.
Snug — but not tight — under the bust, parallel to the floor. In a sports bra or unpadded soft bra is fine; loose bralettes give wrong results.
Across the fullest point of the bust, parallel to the floor. Don't press the tape in.
The widget converts the US result to UK and EU using the modern crosswalk — note that UK D-cup is not the same as US D once you pass DD.
| US | UK | EU |
|---|---|---|
| A | A | A |
| B | B | B |
| C | C | C |
| D | D | D |
| DD | DD | E |
| DDD / E | DDD | F |
| F | E | G |
| G | FF | I |
Used widely before the 1990s — you added 4 (for even-inch bands) or 5 (for odd) to your underbust. It systematically produced loose bands because it was calibrated to corsetry-era materials with no spandex. Modern fabric stretches; the rule doesn't translate.
Specialty fitters (think dedicated lingerie stores rather than department stores) tend to be substantially better than mass-market chains. A 15-minute fitting is the single biggest comfort upgrade most people can make to their wardrobe.