Multiples of bytes
Value Metric
1000 kB kilobyte
10002 MB megabyte
10003 GB gigabyte
10004 TB terabyte
10005 PB petabyte
10006 EB exabyte
10007 ZB zettabyte
10008 YB yottabyte
1024 KiB kibibyte KB kilobyte
10242 MiB mebibyte MB megabyte
10243 GiB gibibyte GB gigabyte
10244 TiB tebibyte
10245 PiB pebibyte
10246 EiB exbibyte
10247 ZiB zebibyte
10248 YiB yobibyte

The kilobyte is a multiple of the unit byte for digital information. The International System of Units (SI) defines the prefix kilo as 1000 (103); therefore one kilobyte is 1000 bytes.[1] The unit symbol for the kilobyte is kB. In information technology, particularly in reference to main memory capacity, kilobyte is traditionally used to denote 1024 (210) bytes. This arises from the powers-of-two sizing common to such memory in digital circuitry. In this context, the symbols K and KB are often used when 1024 bytes is meant.

Definitions and usage

1000 bytes

In the International System of Units (SI) the prefix kilo- means 1000 (103); therefore one kilobyte is 1000 bytes in this system. The unit symbol is kB.

This is the definition recommended by the International Electrotechnical Commission (IEC).[2] This definition, and related definitions of prefixes mega- = 1000000, giga- = 1000000000, etc., are used for data transfer rates[3] in computer networks, internal bus, hard drive and flash media transfer speeds, and for the capacities of most storage media, particularly hard drives,[4] flash-based storage,[5] and DVDs. It is also consistent with the other uses of the SI prefixes in computing, such as CPU clock speeds or measures of performance.

The Mac OS X 10.6 file manager is a notable example of this usage in software. Since Snow Leopard, file sizes are reported with decimal prefixes.[6]

1024 bytes

The kilobyte also often refers to 1024 (210) bytes.[7][8][9] A usage which began when computer engineers began to refer to 1024 bytes as a kilobyte because it was almost the same as the decimal kilobyte of 1000 bytes.[10] This led to corresponding definitions of megabyte and gigabyte as 1048576 (=10242) and 1073741824 (=10243) bytes respectively.

These definitions are used by the Microsoft Windows operating system,[11] which is used on 90% of the world's personal computers.[12] They are also use used for random-access memory capacities, such as main memory and CPU cache sizes, due to the binary addressing of memory.[lower-alpha 1]

It is also used in marketing and billing by some telecommunication companies including Vodafone,[13] AT&T,[14] Orange[15] and Telstra.[16]

The binary representation of 1024 bytes typically uses the symbol KB (uppercase K). The B is often omitted in informal use. For example, a processor with 65,536 bytes of cache might be said to have "64K" of cache.


In December 1998, the IEC addressed such multiple usages and definitions by creating prefixes such as kibi, mebi, gibi, etc., to unambiguously denote powers of 1024.[17] Thus the kibibyte, symbol KiB, represents 210 = 1024 bytes. These prefixes are now part of the International System of Quantities. The IEC further specified that the kilobyte should only be used to refer to 1000 bytes. In practice, kilobyte is still commonly used to refer to 1024 bytes.


See also


  1. Exceptions did exist for machines that used decimal addressing, such as the IBM 1401. A "12 K" 1401 has 12,000 characters of memory.


  1. International Standard IEC 80000-13 Quantities and Units – Part 13: Information science and technology, International Electrotechnical Commission (2008).
  2. Prefixes for Binary Multiples — The NIST Reference on Constants, Units, and Uncertainty
  3. Conversion of Data Transfer Rate Units
  4. 1977 Disk/Trend Report Rigid Disk Drives, published June 1977
  5. SanDisk USB Flash Drive "Note: 1 megabyte (MB) = 1 million bytes; 1 gigabyte (GB) = 1 billion bytes."
  6. "How Mac OS X reports drive capacity". Apple Inc. 2009-08-27. Retrieved 2009-10-16.
  7. Kilobyte – Definition and More from the Free Merriam-Webster Dictionary. (2010-08-13). Retrieved on 2011-01-07.
  8. Kilobyte | Define Kilobyte at (1995-09-29). Retrieved on 2011-01-07.
  9. Definition of kilobyte from Oxford Dictionaries Online. Retrieved on 2011-01-07.
  10. "Prefixes for binary multiples". International Electrotechnical Commission. Retrieved 1 October 2016.
  11. "Determining Actual Disk Size: Why 1.44 MB Should Be 1.40 MB". 2003-05-06. Retrieved 2014-03-25.
  12. "Operating system market share". Retrieved 2016-07-13.
  13. "3G/GPRS data rates". Vodafone Ireland. Retrieved 26 October 2016.
  14. "Data Measurement Scale". AT&T. Retrieved 26 October 2016.
  15. "Internet Mobile Access". Orange Romania. Retrieved 26 October 2016.
  16. "Our Customer Terms" (PDF). Telstra. p. 7. Retrieved 26 October 2016.
  17. National Institute of Standards and Technology. "Prefixes for binary multiples". "In December 1998 the International Electrotechnical Commission (IEC) [...] approved as an IEC International Standard names and symbols for prefixes for binary multiples for use in the fields of data processing and data transmission."
  18. "SA400 minifloppy". 2013-08-14. Retrieved 2014-03-25.
  21. "Determining Actual Disk Size: Why 1.44 MB Should Be 1.40 MB". 2003-05-06. Retrieved 2014-03-25.
  22. "How OS X and iOS report storage capacity". 2013-07-01. Retrieved 2014-03-25.
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