The kilobyte is a multiple of the unitbyte for digital information. The International System of Units defines the prefix kilo as 1000 ; 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 bytes. This arises from the powers-of-two sizing common to memory circuit design. In this context, the symbols KB and K are often used.
The kilobyte has traditionally been used to refer to 1024 bytes, a usage still common. 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 letterK. 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 is equal to one megabyte, 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.
Examples
The Shugart SA-400 5-inch floppy disk held 109,375 bytes unformatted, and was advertised as "110 Kbyte", using the 1000 convention. Likewise, the 8-inch DEC RX01 floppy held 256,256 bytes formatted, and was advertised as "256k". On the other hand, the Tandon 5-inch DD floppy format held 368,640 bytes, but was advertised as "360 KB", following the 1024 convention.
On modern systems, all versions of Microsoft Windows including the newest 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.
