There are 1024 bytes in a KB, 1024 KB in a MB, 1024 MB in a GB so:
872762081280 bytes 1 KB 1 MB 1 GB
x ------- x ------ x ----- = 812 GB
1024 bytes 1024 KB 1024 MB
Note the use of KB, MB, and GB follow the JEDEC memory standards nomenclature
Under IEC 60027 notation these would be denoted by KiB, MiB and GiB
There is much confusion over this since they both use multiples of 1024, but the Decimal nomenclature uses the same KB, MB and GB but with multiples of 1000, as used by some disk manufacturers