Kilobyte

Multiples of bytes
Decimal Value Metric 1000 kB kilobyte 1000^2 MB megabyte 1000^3 GB gigabyte 1000^4 TB terabyte 1000^5 PB petabyte 1000^6 EB exabyte 1000^7 ZB zettabyte 1000^8 YB yottabyte	Binary Value IEC JEDEC 1024 KiB kibibyte KB kilobyte 1024^2 MiB mebibyte MB megabyte 1024^3 GiB gibibyte GB gigabyte 1024^4 TiB tebibyte – 1024^5 PiB pebibyte – 1024^6 EiB exbibyte – 1024^7 ZiB zebibyte – 1024^8 YiB yobibyte –
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 some fields of 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 has been common.

Definitions and usage

1000 bytes

In the International System of Units (SI) the prefix kilo- means 1000 (10³); therefore one kilobyte is 1000 bytes in this system. The unit symbol is kB.

This is the definition recommended by the International Electrotechnical Commission (IEC).^[1] This definition, and related definitions of prefixes mega- = 1000000, giga- = 1000000000, etc., are used for data transfer rates^[2] 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 Mac OS X 10.6 file manager is a notable example of this usage in software. Since Snow Leopard, file sizes are reported with decimal prefixes.^[5]

1024 bytes

In some fields of information technology, the kilobyte instead refers to 1024 (2¹⁰) bytes.^[6][7][8] 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 almost invariably used for random-access memory capacities, such as main memory and CPU cache sizes, due to the binary addressing of memory.^{[lower-alpha 1]} These "binary meanings" of kilobyte, megabyte, etc., are also used by some computer operating systems when reporting disk capacities and file sizes.^[9][10]

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.

Kibibyte

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.^[11] 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, the kilobyte is still commonly used to refer to 1024 bytes.

Examples

• The HP 21MX real-time computer (1974) denoted 196,608 (which is 192×1024) as 196K,^[12] while the HP 3000 business computer (1973) denoted 131,072 (which is 128×1024) as "128K".^[13]
• The Shugart SA-400 5¹⁄₄-inch floppy disk (1976) held 109,375 bytes unformatted,^[14] and was advertised as 110 Kbyte, using the 1000 convention.^[15] Likewise, the 8-inch DEC RX01 floppy (1975) held 256,256 bytes formatted, and was advertised as 256k.^[16] 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.^[17] Conversely, Mac OS X Snow Leopard and newer represent this as 66 kB, rounding to the nearest 1000 bytes.^[18]

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

Notes

1. Exceptions did exist for machines that used decimal addressing, such as the IBM 1401. A "12 K" 1401 has 12,000 characters of memory.

References

1. Prefixes for Binary Multiples — The NIST Reference on Constants, Units, and Uncertainty
2. Conversion of Data Transfer Rate Units
3. 1977 Disk/Trend Report Rigid Disk Drives, published June 1977
4. SanDisk USB Flash Drive "Note: 1 megabyte (MB) = 1 million bytes; 1 gigabyte (GB) = 1 billion bytes."
5. "How Mac OS X reports drive capacity". Apple Inc. 2009-08-27. Retrieved 2009-10-16.
6. Kilobyte – Definition and More from the Free Merriam-Webster Dictionary. Merriam-webster.com (2010-08-13). Retrieved on 2011-01-07.
7. Kilobyte | Define Kilobyte at Dictionary.com. Dictionary.reference.com (1995-09-29). Retrieved on 2011-01-07.
8. Definition of kilobyte from Oxford Dictionaries Online. Askoxford.com. Retrieved on 2011-01-07.
9. "HTG Explains: Why Do Hard Drives Show the Wrong Capacity in Windows?". www.howtogeek.com. Retrieved 2016-04-05. Keep in mind that the hard drive manufacturers are using the accurate description of the terms–the prefix giga, for instance, means a power of 1000, whereas the correct term for powers of 1024 is gibibyte, though it isn’t often used. Unfortunately, Windows has always calculated hard drives as powers of 1024 while hard drive manufacturers use powers of 1000.
10. "For the Same File, Why Windows and Ubuntu Show Different Sizes?". Hectic Geek. Retrieved 2016-04-05. Windows assumes that there are 1024 Bytes in a Kilobyte unit, and 1024 Kilobytes in a Megabyte unit etc.
11. 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."
12. 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
13. Hewlett-Packard (November 1973). "HP 3000 Configuration Guide" (PDF). HP 3000 Computer System and Subsystem Data: pg 59. Retrieved 2010-01-22.
14. "SA400 minifloppy". Swtpc.com. 2013-08-14. Retrieved 2014-03-25.
15. http://www.swtpc.com/mholley/SA400/SA400_Datasheet.pdf
16. http://bitsavers.org/pdf/dec/disc/rx01/EK-RX01-MM-002_maint_Dec76.pdf
17. "Determining Actual Disk Size: Why 1.44 MB Should Be 1.40 MB". Support.microsoft.com. 2003-05-06. Retrieved 2014-03-25.
18. "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.