A binary prefix is a unit prefix for multiples of units in data processing, data transmission, and digital information, notably the bit and the byte, to indicate multiplication by a 12 bits in binary option of 2. The computer industry has historically used the units kilobyte, megabyte, and gigabyte, and the corresponding symbols KB, MB, and GB, in at least two slightly different measurement systems.

In most other contexts, the industry uses the multipliers kilo, mega, giga, etc. The use of the same unit prefixes with two different meanings has caused confusion. By the mid-1960s, binary addressing had become the standard architecture in most computer designs, and main memory sizes were most commonly powers of two. This is the most natural configuration for memory, as all combinations of their address lines map to a valid address, allowing easy aggregation into a larger block of memory with contiguous addresses.

Early computer system documentation would specify the memory size with an exact number such as 4096, 8192, or 16384 words of storage. These are all powers of two, and furthermore are small multiples of 210, or 1024. As storage capacities increased, several different methods were developed to abbreviate these quantities. The method most commonly used today uses prefixes such as kilo, mega, giga, and corresponding symbols K, M, and G, which the computer industry originally adopted from the metric system. 1960 to formalize aspects of the metric system.

The International System of Units does not define units for digital information but notes that the SI prefixes may be applied outside the contexts where base units or derived units would be used. But as computer main memory in a binary-addressed system is manufactured in sizes that were easily expressed as multiples of 1024, kilobyte, when applied to computer memory, came to be used to mean 1024 bytes instead of 1000. This usage is not consistent with the SI. 360 used “1K” to mean 1024. Another style was to truncate the last three digits and append K, essentially using K as a decimal prefix similar to SI, but always truncating to the next lower whole number instead of rounding to the nearest.

If these values had been rounded to nearest they would have become 33K, 66K, and 131K, respectively. This style was used from about 1965 to 1975. In discussions of binary-addressed memories, the exact size was evident from context. For memory sizes of “41K” and below, there is no difference between the two styles.

64K”, “96K”, or “128K” bytes of memory. Capitalization of the letter K became the de facto standard for binary notation, although this could not be extended to higher powers, and use of the lowercase k did persist. The symbols Kbit, Kbyte, Mbit and Mbyte started to be used as “binary units”—”bit” or “byte” with a multiplier that is a power of 1024—in the early 1970s. Real storage capacity is available in 512K increments ranging from 512K to 2,048K bytes. 1048576 bytes could be referred to unambiguously as 1 mebibyte.

The disk drive industry followed a different pattern. Industry practice, more thoroughly documented at Timeline of binary prefixes and continuing today, is generally to specify hard drives using prefixes and symbols with decimal meaning as described by SI. Any block size could be specified up to the maximum track length. Since the block headers occupied space, the usable capacity of the drive was dependent on the block size. Floppy disks for the IBM PC and compatibles quickly standardized on 512-byte sectors, so two sectors were easily referred to as “1K”.