|Orders of magnitude of data|
The International System of Units (SI) defines the prefix kilo as 1000 (103); per this definition, one kilobyte is 1000 bytes. The internationally recommended unit symbol for the kilobyte is kB.
In some areas of information technology, particularly in reference to digital memory capacity, kilobyte instead denotes 1024 (210) bytes. This arises from the prevalence of powers of two in memory circuit design.
Definitions and usage
Base 10 (1000 bytes)
This is the definition recommended by the International Electrotechnical Commission (IEC). This definition, and the related definitions of the prefixes mega (1000000), giga (1000000000), etc., are most commonly used for data transfer rates in computer networks, internal bus, hard drive and flash media transfer speeds, and for the capacities of most storage media, particularly hard drives, flash-based storage, 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 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 (1 MB), where 1 MB is one million bytes.
Base 2 (1024 bytes)
The 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 operating system. Metric prefixes are also used for 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 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, symbol KiB, represents 210 bytes = 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.
- The Shugart SA-400 51⁄4-inch floppy disk (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 51⁄4-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 including the newest (as of 2019) Windows 10 divide by 1024 and represent a 65,536-byte file as "64 KB". Conversely, Mac OS X Snow Leopard and newer represent this as 66 kB, rounding to the nearest 1000 bytes. File sizes are reported with decimal prefixes.
- The binary interpretation is still used in marketing and billing by some telecommunication companies, such as Vodafone, AT&T, Orange and Telstra.
- International Standard IEC 80000-13 Quantities and Units – Part 13: Information science and technology, International Electrotechnical Commission (2008).
- Prefixes for Binary Multiples Archived 2007-08-08 at the Wayback Machine — The NIST Reference on Constants, Units, and Uncertainty
- 1977 Disk/Trend Report Rigid Disk Drives, published June 1977
- SanDisk USB Flash Drive Archived 2008-05-13 at the Wayback Machine "Note: 1 megabyte (MB) = 1 million bytes; 1 gigabyte (GB) = 1 billion bytes."
- Kilobyte – Definition and More from the Free Merriam-Webster Dictionary Archived 2010-04-09 at the Wayback Machine. Merriam-webster.com (2010-08-13). Retrieved on 2011-01-07.
- Kilobyte | Define Kilobyte at Dictionary.com Archived 2010-09-01 at the Wayback Machine. Dictionary.reference.com (1995-09-29). Retrieved on 2011-01-07.
- Definition of kilobyte from Oxford Dictionaries Online Archived 2006-06-25 at the Wayback Machine. Askoxford.com. Retrieved on 2011-01-07.
- "Prefixes for binary multiples". iec.ch. International Electrotechnical Commission. Archived from the original on 25 September 2016. Retrieved 1 October 2016.
- "Determining Actual Disk Size: Why 1.44 MB Should Be 1.40 MB". Support.microsoft.com. 2003-05-06. Archived from the original on 2014-02-09. Retrieved 2014-03-25.
- National Institute of Standards and Technology. "Prefixes for binary multiples". Archived from the original on 2007-08-08. "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."
- "SA400 minifloppy". Swtpc.com. 2013-08-14. Archived from the original on 2014-05-27. Retrieved 2014-03-25.
- "Archived copy" (PDF). Archived from the original (PDF) on 2011-06-08. Retrieved 2011-06-24.CS1 maint: archived copy as title (link)
- "Archived copy" (PDF). Archived from the original (PDF) on 2011-04-23. Retrieved 2011-06-24.CS1 maint: archived copy as title (link)
- "How OS X and iOS report storage capacity". Support.apple.com. 2013-07-01. Archived from the original on 2014-03-04. Retrieved 2014-03-25.
- "How Mac OS X reports drive capacity". Apple Inc. 2009-08-27. Archived from the original on 2009-12-22. Retrieved 2009-10-16.
- "3G/GPRS data rates". Vodafone Ireland. Archived from the original on 26 October 2016. Retrieved 26 October 2016.
- "Data Measurement Scale". AT&T. Retrieved 26 October 2016.[permanent dead link]
- "Internet Mobile Access". Orange Romania. Archived from the original on 26 October 2016. Retrieved 26 October 2016.
- "Our Customer Terms" (PDF). Telstra. p. 7. Archived (PDF) from the original on 10 April 2017. Retrieved 26 October 2016.
- "Terms, Definitions, and Letter Symbols for Microcomputers, Microprocessors, and Memory Integrated Circuits" (PDF). JEDEC Solid State Technology Association. December 2002. Retrieved 22 September 2013.