In dial-up Internet days we surmised that a 56K modem transferred data at 56 000 bits per second (or 56 kbps). Dividing this speed by 10 gave a result of 5.6 kilobytes per second (or 5.6 kBps). Thus, it was easy to calculate download times for files with the following formula, given that their sizes were expressed in kB.
download time in seconds = file size in kB / modem speed in kBpsThis convention was used mostly on bulletin board systems, where users were obsessed with how fast they could download files. It was also common to distinguish between 1000 bytes (1 kB) and 1024 bytes (KB) in writing by using different cases of the letter k.
I continued to use this convention until recently when a reddit post made me aware of IEC 60027-2. This standard, set in 1999, introduced the kibi and disambiguated the units used for expressing data sizes. The blog post ‘The MB Confusion’ describes the standard better than the related Wikipedia article, and explains why its adoption is slow.