Find your bra size from bust and band measurements, with sister sizes.
The band size is your under-bust measurement rounded to the nearest even number. The cup is determined by how many inches your full bust exceeds the band: 1 in = A, 2 in = B, 3 in = C, etc. Sister sizes share the same cup volume: moving up one band size requires moving down one cup letter to keep the same fit, and vice versa. The EU band is your under-bust in centimetres plus roughly 10 cm, rounded to 5.
Band size
Band = under-bust in inches, rounded to nearest even number
Cup letter
Cup index = round(bust inches − band); A=1, B=2, C=3 …
Sister sizes share the same cup volume but use a different band. For example, 34C, 36B, and 32D all have the same amount of cup space. If a 34C doesn't fit well in the band, a 36B gives you more band room with the same cup.
The old 'add 4 inches' method is largely obsolete with modern bra construction. This calculator uses the contemporary approach: round directly to the nearest even number. If you prefer a looser band, size up by 2.
EU band sizes are based on the under-bust in centimetres (plus a fitting allowance), typically 10–15 cm larger than US. So a US/UK 34B is roughly an EU 75B.
Try both and see which gives you a more level band and natural cup shape. Bra fit experts recommend starting with a snug band and adjusting cup size first, since a loose band cannot properly support the bust.