Size reading

I have a 750GB Passport for my Mac, but the computer only says it has 697 GB available for space. why is that? I understand that it has software but not 50GB worth, right??? Please give more insight.

This is not because of the software, the software does not actually take any of the full size

take a look on this Knoledge base article, you will understand it:

And look at the capacity of your internal drive, is the same thing, binary count of the Operating System is different than the decimal count.