How many MB is one GB on a computer?

Basically one KiloByte (KB) has 1024 bytes. Then on MegaByte has 1024 KiloBytes and one GigaByte has 1024 MegaBytes. HDD Manufacturers however put other numbers on disk drives. When they write that disk has e.g. 750 GB it means it has capacity of 750 000 000 000 bytes. But if you backtrace it and multiple it three times with 1024: 750000000000 / (1024*1024*1024) = 698,49 GigaBytes - that's the real capacity of your 750 GB hard drive. So the answer is: one GigaByte has allways 1024 MegaBytes but the capacity of disk is not given in GigaBytes (2^30) but in bilions of bytes (10^9).