Bra size calculator
A 75C and an 80B hold almost the same cup volume: going up a band size and down a cup letter keeps it roughly constant. That is why "sister sizes" are worth knowing: if the cup fits but the band rides up, going down a band and up a cup keeps the volume while tightening the fit. Most of the support comes from the band, not the straps.
Sizing varies considerably between brands and styles. Treat this as a starting point for trying on rather than a definitive size.
Band size comes from the underbust measurement and cup size from the difference between bust and underbust — about 2.5 cm per cup letter. A 75 cm underbust with a 90 cm bust is a 15 cm difference, which is roughly a 75C.
How to measure for a bra
Most of a bra support comes from the band, not the straps, which is why a band that rides up at the back is the clearest sign of a poor fit. The usual correction is to go down a band size and up a cup letter. A sister size, which tightens the band while keeping cup volume the same. Sizing conventions also differ: UK and US cups diverge above D, and EU bands are the underbust in centimetres rounded to the nearest five, so a conversion between systems is never exact.
Questions
Underbust snug and level for the band; bust loose at the fullest point. The difference sets the cup.
A size with roughly the same cup volume on a different band: going up a band and down a cup, or the reverse.
The band is too loose. Go down a band size and up a cup letter to keep the volume while tightening the fit.
Up to D, yes. Above that they diverge; a UK DD is a US E, and it continues from there.
Whenever weight changes noticeably, and roughly annually otherwise. Bands also stretch with wear.