|Multiples of bytes|
|Orders of magnitude of data|
The kilobyte is a multiple of the unit byte for digital information. The International System of Units (SI) defines the prefix kilo as 1000 (103); therefore one kilobyte is 1000 bytes. The unit symbol for the kilobyte is kB. In information technology kilobyte has traditionally also been used to denote 1024 (210) bytes, which arises from binary exponentiation common to digital circuitry. In this context the unit symbol K or KB is common, and invariably used when 1024 bytes are meant.
Definitions and usage
This is the definition recommended by the International Electrotechnical Commission (IEC). This definition, and related definitions of prefixes mega- = 1000000, giga- = 1000000000, etc., are 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.
In certain fields of information technology, the kilobyte instead refers to 1024 (210) bytes. This usage originated as compromise jargon for multiples that needed to be expressed in powers of 2, but lacked a convenient unit prefix. As 1024 (210) approximates 1000 (103), the corresponding SI prefixes were used for the multiples that represent powers of 1024.
This definition, and related definitions of mega = 1048576 (=10242), etc., are mainly used for random-access memory capacities, such as main memory and CPU cache sizes, due to the binary addressing of memory.[a] These "binary meanings" of kilobyte, megabyte, etc., are also used by almost all computer operating systems when reporting disk capacities, memory capacities, and file sizes, with the notable exception being Mac OS X that switched to the decimal definition. Disk drive manufacturers have changed to the decimal definition for their drive capacity in the early 2000s for indicating drive capacity, leading to consumer confusion since their drive capacity was often less than what they expected (see Gigabyte § Consumer confusion). 
The binary representation of 1024 bytes typically uses the symbol KB (uppercase K). The B is often omitted in informal use. For example, a processor with 65,536 bytes of cache might be said to have "64K" of cache.
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 = 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. However, despite this attempt at reducing ambiguity, the kilobyte is still very commonly used to refer to 1024 bytes, and almost exclusively so by IT professionals.
- The HP 21MX real-time computer (1974) denoted 196,608 (which is 192×1024) as 196K, while the HP 3000 business computer (1973) denoted 131,072 (which is 128×1024) as "128K".
- 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 2015) 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.
- 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
- Exceptions did exist for machines that used decimal addressing, such as the IBM 1401. A "12 K" 1401 has 12,000 characters of memory.
- Prefixes for Binary Multiples — The NIST Reference on Constants, Units, and Uncertainty
- Conversion of Data Transfer Rate Units
- 1977 Disk/Trend Report Rigid Disk Drives, published June 1977
- SanDisk USB Flash Drive "Note: 1 megabyte (MB) = 1 million bytes; 1 gigabyte (GB) = 1 billion bytes."
- "How Mac OS X reports drive capacity". Apple Inc. 2009-08-27. Retrieved 2009-10-16.
- Kilobyte – Definition and More from the Free Merriam-Webster Dictionary. Merriam-webster.com (2010-08-13). Retrieved on 2011-01-07.
- Kilobyte | Define Kilobyte at Dictionary.com. Dictionary.reference.com (1995-09-29). Retrieved on 2011-01-07.
- Definition of kilobyte from Oxford Dictionaries Online. Askoxford.com. Retrieved on 2011-01-07.
- Sharma, Kapil; Kabir, Mohammed J.; Norton, Peter C. Norton; Good, Nathan; Steidler-Dennison, Tony (2005). Professional Red Hat Enterprise Linux 3. John Wiley & Sons. p. 134.
Disk manufacturers sell you their disks saying that a kilobyte is 1,000 bytes, that a megabyte is a thousand of those, and that a gigabyte is another thousand of those, giving you 1,000,000,000 bytes to a gigabyte when you buy a disk. The rest of the computer world, including the programmers who write Linux, thinks of a kilobyte as 1,024 bytes (2^10 bytes), a megabyte as 1,048,576 bytes (2^20), and a gigabyte as 1,073,741,824 bytes (2^30), which means that you're buying just a bit less than you might think.
- National Institute of Standards and Technology. "Prefixes for binary multiples". "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."
- Frankenberg, Robert (October 1974). "All Semiconductor Memory Selected for New Minicomputer Series" (PDF). Hewlett-Packard Journal (Hewlett-Packard) 26 (2): pg 15–20. Retrieved 2007-06-18.
196K-word memory size
- Hewlett-Packard (November 1973). "HP 3000 Configuration Guide" (PDF). HP 3000 Computer System and Subsystem Data: pg 59. Retrieved 2010-01-22.
- "SA400 minifloppy". Swtpc.com. 2013-08-14. Retrieved 2014-03-25.
- "Determining Actual Disk Size: Why 1.44 MB Should Be 1.40 MB". Support.microsoft.com. 2003-05-06. Retrieved 2014-03-25.
- "How OS X and iOS report storage capacity". Support.apple.com. 2013-07-01. Retrieved 2014-03-25.
- "Terms, Definitions, and Letter Symbols for Microcomputers, Microprocessors, and Memory Integrated Circuits" (PDF). JEDEC Solid State Technology Association. December 2002. Retrieved 22 September 2013.