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

Multiples of bytes
Value Metric
1000 kBkilobyte
10002 MBmegabyte
10003 GBgigabyte
10004 TBterabyte
10005 PBpetabyte
10006 EBexabyte
10007 ZBzettabyte
10008 YByottabyte
1024 KiBkibibyte KBkilobyte
10242 MiBmebibyte MBmegabyte
10243 GiBgibibyte GBgigabyte
10244 TiBtebibyte
10245 PiBpebibyte
10246 EiBexbibyte
10247 ZiBzebibyte
10248 YiByobibyte

The International System of Units (SI) defines the prefix kilo as 1000 (103); per this definition, one kilobyte is 1000 bytes.[1] The internationally recommended unit symbol for the kilobyte is kB.[1]

In some areas of information technology, particularly in reference to digital memory capacity, kilobyte instead denotes 1024 (210) bytes. This arises from the powers-of-two sizing common to memory circuit design. In this context, the symbols K and KB are often used.

Definitions and usage

Base 10 (1000 bytes)

In the International System of Units (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 (IEC).[2] 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,[3] flash-based storage,[4] 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), a usage still common.[5][6][7] The usage of the metric prefix kilo for binary multiples arose as a convenience, because 1024 is approximately 1000.[8]

The binary interpretation of metric prefixes is still prominently used by the Microsoft Windows operating system,[9] but is deprecated or obsolete in other operating systems. 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.[10] 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.


  • The Shugart SA-400 514-inch floppy disk (1976) held 109,375 bytes unformatted,[11] and was advertised as "110 Kbyte", using the 1000 convention.[12] Likewise, the 8-inch DEC RX01 floppy (1975) held 256,256 bytes formatted, and was advertised as "256k".[13] On the other hand, the Tandon 514-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".[9] Conversely, Mac OS X Snow Leopard and newer represent this as 66 kB, rounding to the nearest 1000 bytes.[14] File sizes are reported with decimal prefixes.[15]
  • The binary interpretation is still used in marketing and billing by some telecommunication companies, such as Vodafone,[16] AT&T,[17] Orange[18] and Telstra.[19]

See also


  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. (2010-08-13). Retrieved on 2011-01-07.
  6. Kilobyte | Define Kilobyte at Archived 2010-09-01 at the Wayback Machine. (1995-09-29). Retrieved on 2011-01-07.
  7. Definition of kilobyte from Oxford Dictionaries Online Archived 2006-06-25 at the Wayback Machine. Retrieved on 2011-01-07.
  8. "Prefixes for binary multiples". 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". 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. "SA400 minifloppy". 2013-08-14. Archived from the original on 2014-05-27. Retrieved 2014-03-25.
  12. "Archived copy" (PDF). Archived (PDF) from the original on 2011-06-08. Retrieved 2011-06-24.CS1 maint: archived copy as title (link)
  13. "Archived copy" (PDF). Archived (PDF) from the original on 2011-07-25. Retrieved 2011-06-24.CS1 maint: archived copy as title (link)
  14. "How OS X and iOS report storage capacity". 2013-07-01. Archived from the original on 2014-03-04. Retrieved 2014-03-25.
  15. "How Mac OS X reports drive capacity". Apple Inc. 2009-08-27. Archived from the original on 2009-12-22. Retrieved 2009-10-16.
  16. "3G/GPRS data rates". Vodafone Ireland. Archived from the original on 26 October 2016. Retrieved 26 October 2016.
  17. "Data Measurement Scale". AT&T. Retrieved 26 October 2016.
  18. "Internet Mobile Access". Orange Romania. Archived from the original on 26 October 2016. Retrieved 26 October 2016.
  19. "Our Customer Terms" (PDF). Telstra. p. 7. Archived (PDF) from the original on 10 April 2017. Retrieved 26 October 2016.
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