The term kilobyte can refer to 1000 bytes (kB), or, with a capital K, Kilobyte means 1024 bytes, two different expressions for multiples of the unit byte used for measuring digital information. While the prefix kilo means 1000, in the computer sciences and the Information Technology field, the term Kilobyte and its abbreviation KB are commonly used to mean 1024 bytes.

The symbol kB is correctly used to refer to 1000 bytes, but central processing unit (CPU) caches always use binary prefixes in which the symbol kB should never appear, which is also the case with random-access memory capacity. The various, often mistakenly-interchanged uses of kb, kB and KB have created great confusion among consumers and many misunderstandings concerning capacity and throughput of computer and Internet-related systems. Technically, the letter B should always be upper case because a lower case b indicates bits, not bytes – so a kb is a kilobit, not a kilobyte.

The base 10 or decimal definition of kB is used in a networking context and as the unit of measure in most forms of data storage media, including Flash-based storage devices and standard computer hard drives. The binary definition KB (or KiB) is used primarily in the context of expressing measures of computer memory.