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The kilobyte is a multiple of the unit
byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...
for digital information. The
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms and initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wid ...
(SI) defines the prefix '' kilo'' as 1000 (103); per this definition, one kilobyte is 1000 bytes.International Standard
IEC 80000-13 ISO 80000 or IEC 80000 is an international standard introducing the International System of Quantities (ISQ). It was developed and promulgated jointly by the International Organization for Standardization (ISO) and the International Electrotech ...
Quantities and Units – Part 13: Information science and technology, International Electrotechnical Commission (2008).
The internationally recommended unit symbol for the kilobyte is kB. In some areas of
information technology Information technology (IT) is the use of computers to create, process, store, retrieve, and exchange all kinds of Data (computing), data . and information. IT forms part of information and communications technology (ICT). An information te ...
, particularly in reference to solid-state memory capacity, ''kilobyte'' instead typically refers to 1024 (210) bytes. This arises from the prevalence of sizes that are powers of two in modern digital memory architectures, coupled with the accident that 210 differs from 103 by less than 2.5%. A
kibibyte The byte is a units of information, unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character (computing), character of text in a computer and for this ...
is defined by Clause 4 of
IEC 80000-13 ISO 80000 or IEC 80000 is an international standard introducing the International System of Quantities (ISQ). It was developed and promulgated jointly by the International Organization for Standardization (ISO) and the International Electrotech ...
as 1024 bytes.


Definitions and usage


Base 10 (1000 bytes)

In the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms and initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wid ...
(SI) the prefix '' kilo'' means 1000 (103); therefore, one kilobyte is 1000 bytes. The unit symbol is kB. This is the definition recommended by the
International Electrotechnical Commission The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic an ...
(IEC).Prefixes for Binary Multiples
— The NIST Reference on Constants, Units, and Uncertainty
This definition, and the related definitions of the prefixes mega (), giga (), etc., are most commonly used for data transfer rates in
computer network A computer network is a set of computers sharing resources located on or provided by Node (networking), network nodes. The computers use common communication protocols over digital signal, digital interconnections to communicate with each ot ...
s, internal bus, hard drive and flash media transfer speeds, and for the capacities of most storage media, particularly
hard drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk platter, platters ...
s, flash-based storage, and
DVD The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital optical disc data storage format. It was invented and developed in 1995 and first released on November 1, 1996, in Japan. The medium can store any k ...
s. It is also consistent with the other uses of the
SI prefix The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms and initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wid ...
es in computing, such as CPU clock speeds or measures of performance. The IEC 80000-13 standard uses the term 'byte' to mean eight bits (1 B = 8 bit). Therefore, 1 kB = 8000 bit. One thousand kilobytes (1000 kB) is equal to one
megabyte The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix ''mega'' is a multiplier of (106) in the International System of Units (SI). Therefore, one megabyte is one million bytes o ...
(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 (210 B). The usage of the metric prefix ''kilo'' for binary multiples arose as a convenience, because 1024 is approximately 1000. The binary interpretation of metric prefixes is still prominently used by the
Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft Microsoft Corporation is an American multinational corporation, multinational technology company, technology corporatio ...
operating system. Metric prefixes are also used for
random-access memory Random-access memory (RAM; ) is a form of computer memory that can be read and changed in any order, typically used to store working Data (computing), data and machine code. A Random access, random-access memory device allows data items to b ...
capacity, such as main memory and
CPU cache A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data (computing), data from the main memory. A cache is a smaller, faster memory, located closer to a proc ...
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 10242 bytes. 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. Thus the
kibibyte The byte is a units of information, unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character (computing), character of text in a computer and for this ...
, symbol KiB, represents 210  bytes = 1024 bytes. These prefixes are now part of the
IEC 80000-13 ISO 80000 or IEC 80000 is an international standard introducing the International System of Quantities (ISQ). It was developed and promulgated jointly by the International Organization for Standardization (ISO) and the International Electrotech ...
standard. The IEC further specified that the kilobyte should only be used to refer to 1000 bytes. The
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms and initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wid ...
restricts the use of the SI prefixes strictly to powers of 10.


Examples

* The Shugart SA-400 5-inch
floppy disk A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined wi ...
(1976) held 109,375 bytes unformatted, and was advertised as "110 Kbyte", using the 1000 convention. Likewise, the 8-inch DEC RX01 floppy (1975) held 256,256 bytes formatted, and was advertised as "256k". On the other hand, the Tandon 5-inch DD floppy format (1978) held 368,640 (which is 360×1024) bytes, but was advertised as "360 KB", following the 1024 convention. * On modern systems, all versions of
Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft Microsoft Corporation is an American multinational corporation, multinational technology company, technology corporatio ...
including the newest ()
Windows 10 Windows 10 is a major release of Microsoft's Windows NT operating system. It is the direct successor to Windows 8.1, which was released nearly two years earlier. It was Software release cycle#Release to manufacturing (RTM), released to ma ...
divide by 1024 and represent a 65,536-byte file as "64 KB". Conversely,
Mac OS X Snow Leopard Mac OS X Snow Leopard (version 10.6) is the seventh software versioning, major release of macOS, Apple Inc., Apple's desktop and server operating system for Macintosh computers. Snow Leopard was publicly unveiled on June 8, 2009 at Apple’s A ...
and newer represent this as 66 kB, rounding to the nearest 1000 bytes. File sizes are reported with decimal prefixes. * the binary interpretation was still used in marketing and billing by some telecommunication companies, such as Vodafone,
AT&T AT&T Inc. is an American multinational corporation, multinational telecommunications holding company headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the world's List of largest companies by revenue, largest telecommunications ...
, Orange and
Telstra Telstra Group Limited is an Australian telecommunications company that builds and operates telecommunications networks and markets voice, mobile, internet access, pay television and other products and services. It is a member of the S&P/ASX 20 ...
.


See also

* History of the floppy disk * Binary prefix * Timeline of binary prefixes * * *


References

* {{Computer Storage Volumes Units of information