When specifying a size of a disk, hard drive manufacturers consider 1 KB to be 1,000 bytes, MB (megabyte) to be 1,000,000 bytes, and gigabyte to be 1,000,000,000 bytes.
However, in software, slightly odd measurement units are used.
KB contains 1024 bytes (210),
1 gigabyte (GB) contains 1,073,741,824 bytes.
The difference grows as sizes grow. For a 750GB hard drive, the discrepancy between advertised and useful capacity is about 7%.
In case of a 1TB drive, the discrepancy is close to 10%.
The differences add up quickly, and although one would reasonably expect a RAID 5 array of eight (8x) 2TB hard drives to be 14TB in size, actually it’s only a miserable 12.6TB.