A flexible magnetic disk consisting of a circular polyester substrate (with a central hole) coated with magnetic oxide and permanently enclosed within a stiff jacket, the inside of which is lined with a cleaning material. The jacket has a radial slot on each side through which the read heads (or head and support pad in the case of a single-sided floppy disk) can contact the disk. As with other magnetic disks, data is recorded in concentric tracks in the magnetic coating; the tracks are divided into sectors. Developing technology has enabled recording density and track density to be raised, thus increasing total storage capacity. A small index hole in the jacket and disk is provided so that a photosensor may be used to generate an index pulse once per revolution. An aperture, the write protect notch, on one edge of the jacket can be blocked to prevent the drive from writing to the disk.

The most common size of floppy disk is 3½ inches. It has a jacket of hard plastic with a metal shutter protecting the read-write slot. Floppy disks typically hold 1.44 megabytes. A former type of 5¼ inch diameter, which was held in a flexible paper envelope, is now obsolete.