Bra sizing has a reputation for being mysterious, and it is not really. It is two measurements and one subtraction. What makes it feel mysterious is that the same two numbers produce a different size depending on whose chart you read, and that most people are measured once, badly, in a shop, and then never again.
What you need
A soft tape measure and a mirror. Do it without a top on, and if you own a bra that fits reasonably well, wear it — measuring over a bra that fits gives a more useful bust number than measuring bare.
The underbust number
Tape directly under the bust, level all the way round, snug enough that it does not slide but not so tight it bites. Breathe out and read it. This is the number that determines the band, and the band is what actually holds the bra up — about eighty per cent of the support comes from it, not from the straps.
The bust number
Across the fullest part, level at the back, loose enough to slide a finger under. Stand up straight. Do not lean forward, whatever a different guide told you: leaning forward gives you a bigger number and a cup that gapes when you stand up.
The subtraction
Bust minus underbust. Roughly 2.5 cm of difference per cup size. Twelve and a half centimetres is around a C, fifteen is around a D. If you land between two, that is not a rounding error — that is genuinely your size, and it is why half cups exist.
The part that matters
A calculated size is a starting point, not an answer. Two bras in the same size from the same maker can fit differently because the cup shape is different. What you are checking for when you try it on is always the same three things: does the band sit level and firm on the loosest hook, does the cup contain without gapping, and are the straps carrying almost no weight. If the answer to the last one is no, go down a band before you touch the cup.