Frank4d has essentially given you an answer - to put it a slightly different way,
Manufacturers advertise the drive sizes where 1 GB = 1 billion bytes. BUT, to a computer, which does math in binary, 1 GB = 2^30 or 1,073,741,824 bytes.
So if we take the manufacturer's standard of 80,000,000,000 bytes and divide by the computer's interpretation, we get
80,000,000,000
-------------- = 74.5 GB to the Computer
1,073,741,824
Note: the actual size often will be slightly more or slightly less because the drive is not going to simply stop making space available at 80 GB/74.5 GB. There's usually a little more usable space or maybe a little less (figure the variation could be as much as a full GB.