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The kilobyte is a multiple of the unit byte for digital information.

The International System of Units (SI) defines the prefix kilo as a multiplication factor of 1000 (10); therefore, 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 random-access memory capacity, kilobyte instead typically refers to 1024 (2) bytes. This arises from the prevalence of sizes that are powers of two in modern digital memory architectures, coupled with the coincidence that 2 differs from 10 by less than 2.5%. A kibibyte is 1024 bytes.

Definitions and usage

Decimal (1000 bytes)

In the International System of Units (SI) the metric prefix kilo means 1,000 (10); therefore, one kilobyte is 1000 bytes. The unit symbol is kB.

This is the definition recommended by the International Electrotechnical Commission (IEC). This definition, and the related definitions of the prefixes mega (1,000,000), giga (1,000,000,000), 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 disk drives, flash-based storage, and DVDs. It is also consistent with the other uses of the metric prefixes in computing, such as CPU clock speeds or measures of performance.

The international standard IEC 80000-13 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.

Binary (1024 bytes)

The term 'kilobyte' has traditionally been used to refer to 1024 bytes (2 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. Binary interpretation is 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 sometimes 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 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 2 bytes = 1024 bytes. These prefixes are now part of IEC 80000-13. 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.

Use of term

• 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.
• Early home computer systems would often advertise using the 1024 convention, hence the naming of the Commodore 64, Commodore 128, and the Amstrad CPC 464.
• 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.
• As of 2016, the binary interpretation was still used in marketing and billing by some telecommunication companies, such as Vodafone, AT&T, Orange and Telstra.

Data examples

"This is an example of a text which is exactly a kilobyte (kB) large. In this case, this text is 10^3 bytes long, but if you were to add 24 extra characters to this string, it would be 2^10 bytes long, which is used in some fields such as information technology. Each character in this string (which includes the quotation marks at the end, by the way) is exactly one byte long, which is 8 bits. The bit is the fundamental unit of information, which represents a single yes or no. So, one could measure the amount of information in a single letter with 5 bits (because 2^5 is 32, there are 26 letters in the English language), but because we also use other characters, like numbers, capital/lowercase letters, symbols (like {}$#*&%!`~), spaces, and more. In a computer's memory, these may be represented with some binary string, such as 01010100 (which represents the letter T), usually this is distinguished from one million ten thousand one hundred by appending '0b' to the front, like 0b01010100."

• The Lord's Prayer, in Latin, is 296 bytes.
• The short story The Cask of Amontillado by Edgar Allan Poe, hosted on Project Gutenberg as an uncompressed plain text file, is 12,843 bytes: this is 12.8 kilobytes (divided by 1,000) or 12.5 kibibytes (divided by 1,024).

• The novel The Picture of Dorian Gray, by Oscar Wilde, hosted on Project Gutenberg as an uncompressed plain text file, is 428,952 bytes; this is 428.95 kilobytes (divided by 1,000) and 418.90 kibibytes (divided by 1,024). Great Expectations is 994,639 bytes, and Moby Dick is 1,191,763 bytes.

See also

• History of the floppy disk
• Binary prefix
• Timeline of binary prefixes
• Gigabyte § Consumer confusion
• JEDEC memory standards § Unit prefixes for semiconductor storage capacity
• Units of information § Size examples

References

1. ^ International Standard IEC 80000-13 Quantities and Units – Part 13: Information science and technology, International Electrotechnical Commission (2008).
2. Prefixes for Binary Multiples Archived 2007-08-08 at the Wayback Machine — The NIST Reference on Constants, Units, and Uncertainty
3. 1977 Disk/Trend Report Rigid Disk Drives, published June 1977
4. 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."
5. 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.
6. 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.
7. Definition of kilobyte from Oxford Dictionaries Online Archived 2006-06-25 at the Wayback Machine. Askoxford.com. Retrieved on 2011-01-07.
8. "Prefixes for binary multiples". iec.ch. International Electrotechnical Commission. Archived from the original on 25 September 2016. Retrieved 1 October 2016.
9. ^ "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.
10. 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."
11. The International System of Units (PDF) (9th ed.), International Bureau of Weights and Measures, Dec 2022, p. 143, ISBN 978-92-822-2272-0. "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 "
12. "SA400 minifloppy". Swtpc.com. 2013-08-14. Archived from the original on 2014-05-27. Retrieved 2014-03-25.
13. "Archived copy" (PDF). Archived from the original (PDF) on 2011-06-08. Retrieved 2011-06-24.{{cite web}}: CS1 maint: archived copy as title (link)
14. "Archived copy" (PDF). Archived from the original (PDF) on 2011-04-23. Retrieved 2011-06-24.{{cite web}}: CS1 maint: archived copy as title (link)
15. "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.
16. "How Mac OS X reports drive capacity". Apple Inc. 2009-08-27. Archived from the original on 2009-12-22. Retrieved 2009-10-16.
17. "3G/GPRS data rates". Vodafone Ireland. Archived from the original on 26 October 2016. Retrieved 26 October 2016.
18. "Data Measurement Scale". AT&T. Retrieved 26 October 2016.
19. "Internet Mobile Access". Orange Romania. Archived from the original on 26 October 2016. Retrieved 26 October 2016.
20. "Our Customer Terms" (PDF). Telstra. p. 7. Archived (PDF) from the original on 10 April 2017. Retrieved 26 October 2016.
21. "Vulgata Clementina". Archived from the original on 2022-12-05. Retrieved 2024-02-29. pater noster qui es in cælis sanctificetur nomen tuum adveniat regnum tuum fiat voluntas tua sicut in cælo et in terra panem nostrum supersubstantialem da nobis hodie et dimitte nobis debita nostra sicut et nos dimittimus debitoribus nostris et ne nos inducas in tentationem sed libera nos a malo
22. Poe, Edgar Allan. "The Cask of Amontillado". Archived from the original on 2024-05-08. Retrieved 2024-02-29 – via Project Gutenberg.
23. Dickens, Charles (July 1, 1998). Great Expectations. Archived from the original on February 29, 2024. Retrieved February 29, 2024 – via Project Gutenberg.
24. Melville, Herman (July 1, 2001). Moby Dick; Or, The Whale. Archived from the original on February 29, 2024. Retrieved February 29, 2024 – via Project Gutenberg.

• "Terms, Definitions, and Letter Symbols for Microcomputers, Microprocessors, and Memory Integrated Circuits" (PDF). JEDEC Solid State Technology Association. December 2002. Archived (PDF) from the original on 13 March 2016. Retrieved 22 September 2013.

• Units of information