Hard disk drives HDD sizes are measured by HDD manufactures in decimal . So when they say a HDD is 750 GB in size, they do really mean that it is 750 000 000 000 Bytes. Giga means billion, that’s why. HDD sizes are measured by Windows and Mac (as of writing) in binary. So a 750 GB is shown up as 698.49 in Windows 7 or Mac OS X Leopard. 698.49 is actually 698.49 GiB, but they mistakenly show it as 698.49 GB. This is where the most confusion confusion is coming from. Most Linux distributions, however, show it correctly as 698.49 GiB. RAM Random Access Memory is measured in binary . So they are actually 2 GiB, 4 GiB etc. Manufactures label them incorrectly as GB. Windows and Mac OS X also label them incorrectly as GB. Files File sizes in computers are measured in binary . Windows and Mac OS X measure them in binary but show the decimal unit. Linux does it correctly by measuring in binary and displaying them in binary units also. CDs These are measured in binary by the manufacture