UK 40DD Bra Size to Australian

A bra size is two measurements pretending to be one label. The band comes from your underbust, and the cup comes from the difference between your bust and your underbust — roughly one inch per cup. Every country writes the same pair differently. The band is the easy half: it is the underbust in centimetres in Europe and Japan, that same number plus fifteen in France and Spain, a small index in Italy, and a number of its own in the US, UK and Australia — so a 75 band is a US 34, a French 90, an Italian 2 and an Australian 12, all describing roughly a 75‑cm underbust. The cup is where it goes wrong. Cups convert by position, not by letter: the fifth cup is a DD in the US and UK but an E in Europe, and the sixth is a DDD in the US, an E in the UK and an F in Europe. That single fact is the most expensive misunderstanding in lingerie — a UK E is not an EU E; it is an EU F. Note too that the US and UK band number is not your underbust measurement in inches: it already has roughly four inches built in, which is why a 34 band suits a 29–30‑inch underbust. Measure snugly under the bust for the band and loosely across the fullest part for the bust, take the difference, and treat the answer as a starting point — brands differ, and the same pair of numbers can fit two ways.