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Is It Really 1 TB?

Hitachi's 7K1000 Terabyte Hard Drive
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1 TG is not always 1 TB: Hard drive manufacturers count one kilobyte as 1,000 bytes, which results in a total capacity of 953,867 MB or 935.5 GB.

There are two different ways of counting bits and bytes. If you apply international standards such as the decimal system, one kilo equals a factor of 1,000. In information technology, where the binary approach is the basis, one kilobyte equals 1,024 or 210 bytes. This difference of 2.4% multiplies as you go from kilobytes up to megabytes, gigabytes and then terabytes; the result is a substantial difference in overall capacity.

Windows and other operating systems typically use the binary approach, while hard drive manufacturers and everything outside IT rely on the decimal system. Of course this is very much self-serving for hard drive manufacturers, because it leads to larger advertised capacities. At the same time, you could argue that nothing else except capacities in IT are based on the binary systems, and that it may be more difficult to comprehend.

The IEEE (Institute of Electrical and Electronics Engineers) introduced an "Amendment to IEC International Standard IEC 60027-2" in 2000, which describes a clear way to differentiate between the decimal and the binary system when talking about bytes: Kibi (Ki), Mebi (Mi), Gibi (Gi), Tebi (Ti), Pebi (Pi) represent kilo, mega, giga, tera and peta binaries, which says that...

  • one Kibi byte (1 KiB) has 1,024 bytes,
  • one Mebi byte (1 MiB) has 1,024 KiB,
  • one Gibi byte (GiB) has 1,024 MiB
  • etc.

Simply said, the binary way of data information carries an "i" between the unit symbol and the capital B, which represents the byte.

Prefix Binary/IEEE Symbol Binary Meaning Decimal Prefix Decimal Symbol Decimal Meaning Conversion
Kibi Ki 210 Kilo k 10³ 1 KiB = 1.02 kB
1 kB = 0.977 KiB
Mebi Mi 220 Mega M 106 1 MiB = 1.05 MB
1 MB = 0.954 MiB
Gibi Gi 230 Giga G 109 1 GiB = 1.07 GB
1 GB = 0.932 GiB
Tebi Ti 240 Tera T 1012 1 TiB = 1.10 TB
1 TB = 0.909 TiB
Pebi Pi 250 Peta P 1015 1 PiB = 1.13 PB
1 PB = 0.888 PiB

As you can see from the table above, one decimal terabyte equals 0.909 binary gigabytes. This is the capacity that your operating system will display.

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