View source for Kilobyte

{{Short description|Multiple of the unit byte}}
{{Quantities of bytes}}
The '''kilobyte''' is a multiple of the unit [[Byte|byte]] for [[Computer data storage|digital information]].
The [[International System of Units]] (SI) defines the prefix ''[[Kilo-|kilo]]'' as a multiplication factor of 1000 (10<sup>3</sup>); therefore, one kilobyte is 1000 bytes.<ref name="IEC80000">International Standard IEC 80000-13 Quantities and Units – Part 13: Information science and technology, International Electrotechnical Commission (2008).</ref> The internationally recommended unit symbol for the kilobyte is '''kB'''.<ref name="IEC80000" />

In some areas of [[Engineering:Information technology|information technology]], particularly in reference to solid-state memory capacity, ''kilobyte'' instead typically refers to 1024 (2<sup>10</sup>) bytes. This arises from the prevalence of sizes that are powers of two in modern digital memory architectures, coupled with the coincidence that 2<sup>10</sup> differs from 10<sup>3</sup> by less than 2.5%. A [[Kibibyte|kibibyte]] is defined by IEC 80000-13 as 1024 bytes.


==Definitions and usage==
===Base 10 (1000 bytes)===
In the [[International System of Units]] (SI) the prefix ''[[Kilo-|kilo]]'' means 1000 (10<sup>3</sup>); therefore, one kilobyte is 1000 bytes. The unit symbol is kB.

This is the definition recommended by the [[Organization:International Electrotechnical Commission|International Electrotechnical Commission]] (IEC).<ref name="NIST">[http://physics.nist.gov/cuu/Units/binary.html Prefixes for Binary Multiples] {{webarchive|url=https://web.archive.org/web/20070808000831/http://physics.nist.gov/cuu/Units/binary.html |date=2007-08-08 }} — The NIST Reference on Constants, Units, and Uncertainty</ref>
This definition, and the related definitions of the prefixes [[Mega-|mega]] ({{gaps|1|000|000}}), [[Giga-|giga]] ({{gaps|1|000|000|000}}), etc., are most commonly used for data transfer rates in [[Computer network|computer network]]s, internal bus, hard drive and flash media transfer speeds, and for the capacities of most storage media, particularly hard drives,<ref>1977 Disk/Trend Report Rigid Disk Drives, published June 1977</ref> [[Flash memory|flash]]-based storage,<ref>[http://apac.sandisk.com/Products/Catalog(1349)-SanDisk_Extreme_Ducati_Edition_USB_Flash_Drive.aspx SanDisk USB Flash Drive] {{webarchive|url=https://web.archive.org/web/20080513155718/http://apac.sandisk.com/Products/Catalog%281349%29-SanDisk_Extreme_Ducati_Edition_USB_Flash_Drive.aspx |date=2008-05-13 }} "Note: 1 megabyte (MB) = 1 million bytes; 1 gigabyte (GB) = 1 billion bytes."</ref> and [[Engineering:DVD|DVD]]s. It is also consistent with the other uses of the [[SI prefix]]es in computing, such as [[Hertz#Computers|CPU clock speeds]] or [[FLOPS|measures of performance]].

The IEC 80000-13 standard uses the term 'byte' to mean eight [[Bit|bit]]s (1 B = 8 bit).  Therefore, 1 kB = 8000 bit. One thousand kilobytes (1000 kB) is equal to one [[Megabyte|megabyte]] (1 MB), where 1 MB is one million bytes.

===Base 2 (1024 bytes)===
The term 'kilobyte' has traditionally been used to refer to 1024 bytes (2<sup>10</sup> B).<ref>[http://www.merriam-webster.com/dictionary/kilobyte Kilobyte – Definition and More from the Free Merriam-Webster Dictionary] {{webarchive|url=https://web.archive.org/web/20100409171320/http://www.merriam-webster.com/dictionary/kilobyte |date=2010-04-09 }}. Merriam-webster.com (2010-08-13). Retrieved on 2011-01-07.</ref><ref>[http://dictionary.reference.com/browse/kilobyte Kilobyte | Define Kilobyte at Dictionary.com] {{webarchive|url=https://web.archive.org/web/20100901202451/http://dictionary.reference.com/browse/kilobyte |date=2010-09-01 }}. Dictionary.reference.com (1995-09-29). Retrieved on 2011-01-07.</ref><ref>[http://www.askoxford.com/concise_oed/kilobyte?view=uk Definition of kilobyte from Oxford Dictionaries Online] {{webarchive|url=https://web.archive.org/web/20060625132017/http://www.askoxford.com/concise_oed/kilobyte?view=uk |date=2006-06-25 }}. Askoxford.com. Retrieved on 2011-01-07.</ref> The usage of the metric prefix ''kilo'' for binary multiples arose as a convenience, because 1024 is approximately 1000.<ref>{{cite web|title=Prefixes for binary multiples|url=http://www.iec.ch/si/binary.htm|website=iec.ch|publisher=International Electrotechnical Commission|access-date=1 October 2016|url-status=live|archive-url=https://web.archive.org/web/20160925125914/http://www.iec.ch/si/binary.htm|archive-date=25 September 2016}}</ref>

The binary interpretation of metric prefixes is still prominently used by the [[Software:Microsoft Windows|Microsoft Windows]] operating system.<ref name="auto">{{cite web |url=http://support.microsoft.com/kb/121839 |title=Determining Actual Disk Size: Why 1.44 MB Should Be 1.40 MB |publisher=Support.microsoft.com |date=2003-05-06 |access-date=2014-03-25 |url-status=live |archive-url=https://web.archive.org/web/20140209012305/http://support.microsoft.com/kb/121839 |archive-date=2014-02-09 }}</ref> Metric prefixes are also used for [[Random-access memory|random-access memory]] capacity, such as main memory and [[CPU cache]] size, due to the prevalent binary addressing of memory.

The binary meaning of the kilobyte for 1024 bytes typically uses the symbol KB, with an uppercase letter ''K''. The ''B'' is often omitted in informal use. For example, a processor with 65,536 bytes of cache memory might be said to have "64 K" of cache. In this convention, one thousand and twenty-four kilobytes (1024 KB) is equal to one megabyte (1 MB), where 1 MB is 1024<sup>2</sup> bytes.

In December 1998, the [[Organization:International Electrotechnical Commission|IEC]] addressed such multiple usages and definitions by creating prefixes such as kibi, mebi, gibi, etc., to unambiguously denote powers of 1024.<ref>{{cite web|url=http://physics.nist.gov/cuu/Units/binary.html|title=Prefixes for binary multiples|author=National Institute of Standards and Technology|url-status=live|archive-url=https://web.archive.org/web/20070808000831/http://physics.nist.gov/cuu/Units/binary.html|archive-date=2007-08-08}} "In December 1998 the [[Organization:International Electrotechnical Commission|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."</ref> Thus the [[Kibibyte|kibibyte]], symbol KiB, represents 2<sup>10</sup>&nbsp; bytes = 1024 bytes. These prefixes are now part of the IEC 80000-13 standard. The IEC further specified that the kilobyte should only be used to refer to 1000 bytes.  The [[International System of Units]] restricts the use of the SI prefixes strictly to powers of 10.<ref>{{SIbrochure9th|page=143}}. "The SI prefixes refer strictly to powers of 10. They should not be used to indicate powers of 2 (for example, one kilobit represents 1000 bits and not 1024 bits). The names and symbols for prefixes to be used with powers of 2 are recommended as follows: kibi Ki 2<sup>10</sup> [...]"</ref>

==Examples==
* The [[Company:Shugart Associates|Shugart]] SA-400 5{{1/4}}-inch [[Floppy disk|floppy disk]] (1976) held 109,375 bytes unformatted,<ref>{{cite web |url=http://www.swtpc.com/mholley/SA400/SA400_Index.htm |title=SA400 minifloppy |publisher=Swtpc.com |date=2013-08-14 |access-date=2014-03-25 |url-status=dead |archive-url=https://web.archive.org/web/20140527094602/http://www.swtpc.com/mholley/SA400/SA400_Index.htm |archive-date=2014-05-27 }}</ref> and was advertised as "110 Kbyte", using the 1000 convention.<ref>{{cite web |url=http://www.swtpc.com/mholley/SA400/SA400_Datasheet.pdf |title=Archived copy |access-date=2011-06-24 |url-status=dead |archive-url=https://web.archive.org/web/20110608195322/http://www.swtpc.com/mholley/SA400/SA400_Datasheet.pdf |archive-date=2011-06-08 }}</ref>  Likewise, the 8-inch [[Company:Digital Equipment Corporation|DEC]] RX01 floppy (1975) held 256,256 bytes formatted, and was advertised as "256k".<ref>{{cite web |url=http://bitsavers.org/pdf/dec/disc/rx01/EK-RX01-MM-002_maint_Dec76.pdf |title=Archived copy |access-date=2011-06-24 |url-status=dead |archive-url=https://web.archive.org/web/20110423194129/http://www.bitsavers.org/pdf/dec/disc/rx01/EK-RX01-MM-002_maint_Dec76.pdf |archive-date=2011-04-23 }}</ref>  On the other hand, the [[Company:Tandon Corporation|Tandon]] 5{{1/4}}-inch [[Double density|DD]] floppy format (1978) held 368,640 (which is 360×1024) bytes, but was advertised as "360 KB", following the 1024 convention.
* Early home computer systems would often advertise using the 1024 convention, hence the naming of the [[Engineering:Commodore 64|Commodore 64]], Commodore 128, and the Amstrad CPC 464.
* On modern systems, all versions of [[Software:Microsoft Windows|Microsoft Windows]] including the newest ({{as of|2019|lc=y}}) [[Software:Windows 10|Windows 10]] divide by 1024 and represent a 65,536-byte file as "64&nbsp;KB".<ref name="auto"/> Conversely, [[Software:Mac OS X Snow Leopard|Mac OS X Snow Leopard]] and newer represent this as 66&nbsp;kB, rounding to the nearest 1000 bytes.<ref>{{cite web |url=http://support.apple.com/kb/TS2419 |title=How OS X and iOS report storage capacity |publisher=Support.apple.com |date=2013-07-01 |access-date=2014-03-25 |url-status=live |archive-url=https://web.archive.org/web/20140304015219/http://support.apple.com/kb/TS2419 |archive-date=2014-03-04 }}</ref> File sizes are reported with decimal prefixes.<ref>{{cite web |url=http://support.apple.com/kb/TS2419 |title=How Mac OS X reports drive capacity |publisher=Apple Inc |date=2009-08-27 |access-date=2009-10-16 |url-status=live |archive-url=https://web.archive.org/web/20091222235620/http://support.apple.com/kb/TS2419 |archive-date=2009-12-22 }}</ref>
* {{As of|2016|post=,}} the binary interpretation was still used in marketing and billing by some telecommunication companies, such as [[Company:Vodafone|Vodafone]],<ref>{{cite web|publisher=Vodafone Ireland|title=3G/GPRS data rates|url=https://www.vodafone.ie/planscosts_bus/data/|access-date=26 October 2016|url-status=live|archive-url=https://web.archive.org/web/20161026232743/https://www.vodafone.ie/planscosts_bus/data/|archive-date=26 October 2016}}</ref> AT&T,<ref>{{cite web |publisher=AT&T |title=Data Measurement Scale |url=http://www.att.com/support_static_files/KB/KB24648.html |access-date=26 October 2016 }} </ref> [[Company:Orange S.A.|Orange]]<ref>{{cite web|publisher=Orange Romania|title=Internet Mobile Access|url=https://www.orange.ro/recharge/internet.html|access-date=26 October 2016|url-status=dead|archive-url=https://web.archive.org/web/20161026234911/https://www.orange.ro/recharge/internet.html|archive-date=26 October 2016}}</ref> and [[Company:Telstra|Telstra]].<ref>{{cite web|publisher=Telstra|title=Our Customer Terms|page=7|url=https://www.telstra.com.au/content/dam/tcom/personal/consumer-advice/pdf/intl-roaming.pdf|access-date=26 October 2016|url-status=live|archive-url=https://web.archive.org/web/20170410020355/https://www.telstra.com.au/content/dam/tcom/personal/consumer-advice/pdf/intl-roaming.pdf|archive-date=10 April 2017}}</ref>

==See also==
{{cols}}
* History of the floppy disk
* [[Binary prefix]]
* [[Timeline of binary prefixes]]
* {{Section link|Gigabyte|Consumer confusion}}
* {{Section link|JEDEC memory standards|Unit prefixes for semiconductor storage capacity}}
* {{Section link|Units of information|Size examples}}
{{colend}}

==References==
{{Reflist|30em}}
* {{cite web |url=http://www.jedec.org/sites/default/files/docs/JESD100B01.pdf |title=Terms, Definitions, and Letter Symbols for Microcomputers, Microprocessors, and Memory Integrated Circuits |date=December 2002 |publisher=JEDEC Solid State Technology Association |access-date=22 September 2013}}

{{Computer Storage Volumes}}

[[Category:Units of information]]

{{Sourceattribution|Kilobyte}}