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A exabyte is a unit of measurement in computers of approximately one million million million (American quintillion) bytes.

Because of irregularities in definition and usage of the kilobyte, the exact number could be any one of the following:

  1. 1,152,921,504,606,846,976 bytes - 1024 times 1024 times 1024 times 1024 times 1024 times 1024, or 260. This is 1024 times a petabyte. This is the definition used in computer science and computer programming
  2. 1,000,000,000,000,000,000 bytes - or 10 18.
See integral data type.

A zettabyte is 1024 times an exabyte.

As of 2003, exabytes of data are almost never encountered in a practical context. For example the total amount of printed material in the world is estimated to be around a fifth of an exabyte. Exabytes may also appear to be encountered if a computer's file system is corrupt and displaying incorrect file sizes. However, one may hear of 16 exabytes (or 18 times 1018) of address space when discussing various 64-bit architectures.

To clarify the meaning (1) above, the International Electrotechnical Commission (IEC), a standards body, in 1998 defined new prefixes by combining the International System of Units (SI) prefixes with the word "binary" (see Binary prefix). Thus meaning (1) is called by the IEC a exbibyte (EiB), and meaning (2) is called by the IEC an exabyte. This naming convention has not, as of 2003, been widely adopted.

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Exabyte is also the name used for a brand of digital tape cartridges from NCR Corporation. Of course no-one has yet made a cartridge capable of holding a true exabyte.