>>15144495USA count cup sizes per inches of band-bust difference and often (but not always, depends on manufacturer) include double letters like DD. It also starts at AA for a zero-inch difference.
UK is similar but tends to use double letters more often and starts at A.
This means UK cup sizes tend to be bigger because it'll go A-B-C-D-DD-E-F-FF etc where US would use AA-AA-B-C-D-DD-E-F etc.
But it really depends on the manufacturer.
Japan on the other hand counts cup sizes as 2.5cm of bust-band difference and start at AA for 7.5cm difference.
This means they start at a smaller letter but the cup sizes grow faster because of the 0.04cm difference per cup size and also they don't use double letters past A.
So UK G cup is (usually) about 31-32cm of band-bust difference, US G cup is about 29cm (one cup less) and japanese G cup is about 25cm.
