kB to MB Converter: From Floppy Disks to Modern Webpages

Floppy Disks: When Portable Storage Was Measured in Kilobytes

A floppy disk is a removable magnetic disk: a thin, flexible disk coated with magnetic material spins inside a drive, and a head writes and reads patterns that stand for data. The first 8-inch and later 5.25-inch disks lived in flexible jackets, which is where the word floppy comes from. The 3.5-inch version kept a flexible disk inside a rigid case with a sliding shutter, but the name stuck.

From Punched Cards to a Disk You Could Mail

IBM began work on the floppy in 1967. A small team led by David L. Noble was asked for an inexpensive way to load code into the IBM 3330 mainframe storage controller, and the result, code-named Minnow, shipped as the 23FD in 1971: an 8-inch, read-only disk holding about 80 kB, which the Computer History Museum and IBM both equate to roughly 3,000 punched cards. The disk was small enough to post, where cards needed a box. Later versions added writing, and the technology was patented in 1972. The Computer History Museum's 1971 entry and IBM's history of floppy-disk storage tell the fuller story.

Eight-inch equipment was too big for desktop computers. Shugart Associates answered a request from Wang Laboratories for a cheaper drive with the 5.25-inch SA 400 "Minifloppy", announced in 1976 at 110 kB. More tracks, a second side and denser recording pushed 5.25-inch capacities to 360 kB and, in 1984, to the 1.2 MB high-density disk that IBM introduced for the PC. The Computer History Museum's Minifloppy entry records the 1976 drive and notes that in 1982 the Microfloppy Industry Committee agreed a 3.5-inch standard based on a Sony design. IBM says it brought out the 3.5-inch disk with 1.44 MB of storage two years after the 1.2 MB disk.

Why the "1.44 MB" Label Does Not Add Up

The arithmetic starts from the disk's physical layout. A high-density 3.5-inch disk has 80 tracks on each of two sides, 18 sectors on each track and 512 bytes in each sector:

The label divides by 1,024 once and by 1,000 once, so its "megabyte" is 1,024,000 bytes. That is neither the decimal megabyte of 1,000,000 bytes nor the binary mebibyte of 1,048,576 bytes, which is why two sources can quote the same disk's capacity differently. It is also why this page marks the floppy rows in the table above as labels rather than exact conversions.

From Sneakernet to the Save Icon

The floppy let software companies sell programs by post or in shops, and before networks reached most computers people used disks to carry files between machines, a habit called "sneakernet". As programs grew, installs needed more disks: IBM notes that the diskette version of Windows 95 came on 13 diskettes loaded one at a time. At the mid-1990s peak IBM puts sales above 5 billion disks a year.

Then capacity, speed and reliability overtook the format. Recordable CDs, USB drives, memory cards and network transfers took its place, and by the mid-2000s floppies had largely vanished. The icon survived. Many programs still show a floppy disk for "Save", including to people who have never held one.