blu ray real capacity

[xlr8]

hi

i have a question about the real capacity of blu ray discs

the faq says it is 25 gb, but it is not clear, there is big confusion about what is a gb, if 1024^3 or 1000^3, and obviously, in fact, the blu ray disc capacity measured from software is not any of the two

it seems i can not record more than approx 24,220,000,000 bytes

i expected it was 25 billions exactly, not so less

in fact dvds are told 4.7 gb and they are 4,700,000,000

so, it is me having problems or it is true blu ray discs can store only 24 billions and few more bytes?

my system xp sp3, writer lg ggw h20l with latest firmware
programs tried: nero 8, ashampo burning studio 8, imgburn 2.4

thx