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Why is a kilobyte 1,024 bytes rather than 1,000?

Why is a kilobyte 1,024 bytes rather than 1,000?

In the measurement units system, the prefix "kilo" is to multiply by 1,000, "mega" to multiply by 1,000,000 end "giga" to multiply by 1,000,000,000.

These are powers of 10:

  • 1,000 is 10 to the power 3,
  • 1,000,000 is 10 to the power 6
  • and 1,000,000,000 is 10 to the power 9.

Also, 1,000,000 = 1,000 x 1,000 and 1,000,000,000 = 1,000 x 1,000,000, so that a mega is 1,000 kilo and a giga is 1,000 mega.

But in usual computer science, it is not so: a kilobyte is 1,024 bytes rather than 1,000 bytes.

This is because in computer science, the numeration is done in 2 basis ('0' and '1' are the only digits) rather than the 10 basis, so that the powers of 2 are preferred to the powers of 10.

And 1,024 is a power of 2, it is 2 to the power 10. More precisely, it happens to be a power of 2 that is close to 10 to the power 3: that's why it is used for the definition of a kilobyte.

The convention is the same for the megabyte, that is 1,024 kilobytes, or 1,024 x 1,024 = 1,048,576 bytes. The multiplier is also a power of 2, it is 2 to the power 20, that is close to 10 to the power 6, or 1 million.

And the gigabyte is 1,024 megabytes, that is 1,024 x 1,024 x 1,024 = 1,073,741,824 bytes. The number of bytes in a gigabyte is a power of 2, it is 2 to the power 30, that is close to 10 to the power 9, or 1 billion.

Note that the official standard definition of a kilobyte is indeed 1,000 bytes, the usual definition as 1,024 bytes is standardized as a "kibibyte".

But this is the official definition only, not the usual one… except for sellers of memory devices, for whom the official definition gives a bigger amount of memory: an "official" gigabyte is 7.37% more than a "regular" one…

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