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The band is your underbust measurement rounded to the nearest even number; bra bands come in even sizes only. The cup is the difference between bust and band: one inch is an A, two a B, three a C, and so on up the alphabet.
Sister sizes matter more than most charts admit. A 36C holds the same cup volume as a 34D or a 38B; only the band length changes. If a bra fits in the cup but rides up at the back, go down a band and up a cup, not just down a band.
Two caveats. First, this gives a starting size, not a verdict: cut, stretch and padding all move the goalposts, so check the size notes on the product page too. Second, measure on a plain day; measurements taken over a padded bra run about an inch high.
If the result surprises you, read the five signs your current size is wrong. Most people who remeasure after years in one size find at least two of them.