Kilobyte (KB)

One thousand bytes under the SI definition, though for decades it was used loosely to mean 1,024.

The kilobyte is where the whole confusion starts. Kilo means a thousand everywhere else in measurement, but computer memory is built in powers of two, and 2 to the power of 10 comes out at 1,024 — near enough a thousand that early engineers simply called it one. The approximation held while the numbers were small. By the time storage reached gigabytes the gap had widened to seven per cent, which is why a hard drive sold as 500 GB shows up as about 465 in Windows: the manufacturer counts in thousands and the operating system counts in 1,024s. The IEC settled it in 1998 by inventing the kibibyte for the binary value and leaving the kilobyte at a clean 1,000. Hardly anyone uses the new words, so the ambiguity continues.