If the actual size of a floppy disk is 1.47 MB then why does the floppy disk only show 1.44 MB of space?

Answer 1

Floppy disk is 1.47 MB but shows 1.44 MB of space... This is due to an inconsistency with measurement standards; specifically that of decimal (base 10) versus binary (base 2). 1,440 kB is actually ~1.47 MB because there are 1,024 bytes in a kB; 1,440 kB x 1024 1,474,560 bytes ~1.47 MB.

Sidenote:

One might think that the 1.47 MB is indeed the "raw" capacity as opposed to the "formatted" capacity. And, that the formatting itself requires 0.03MB which is used on Windows for the File Allocation Table or FAT.

But...

An unformatted 3½" floppy drive is actually 2.0 MB, so the 1.44-1.47 discrepancy has nothing to do with formatting overhead.

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Answer 2

Two reasons why there is rarely (never) an exact match between the size of a disk printed on the box and the size reported by the operating system (OS):

• The OS keeps some space for itself, to keep track of which files are stored where on the disk. This space is left out when the OS reports disk size, as it is not available to the user.
• There are two ways of calculating the size of a disk. One method says that 1 MB is 1000*1000=1,000,000 bytes (decimal counting, used by disk manufacturers), the other says 1 MB is 1024*1024=1,048,576 bytes (binary counting, used by the operating system).

This applies not only to floppy disks, but also to hard disks, memory sticks and other storage media.

The larger the disk, the larger the difference between the size reported by the manufacturer and the size reported by the OS.