Computer storage density is a measure of the quantity of information bits that can be stored on a given length of track, area of surface, or in a given volume; of a computer storage medium. Generally, higher density is more desirable, for it allows greater volumes of data to be stored in the same physical space. Density therefore has a direct relationship to storage capacity of a given medium.

For example, the capacity of a standard 3½ inch "high density" ("1.44 megabyte") floppy disk (diskette) is 11,796,480 bits (using MS-DOS format). The recording density could be measured in these ways:

• Linear recording density: The data is stored on concentric tracks along the surface of the diskette. The maximum recording density along 1 inch of the innermost track is 17,434 bits per inch (686 bits per millimetre).

• Surface density: The diskette will store 80 tracks of data at a spacing of 135 tracks per inch (5.05 tracks per millimeter) with the innermost track at approximately 10.5 millimeters radius and the outermost track is at approximately 25.5 millimeters radius. Therefore, the tracks occupy a surface storage area of approximately 1700 square millimeters. The effective recording density is therefore 11,796,480/1,700 or 6,940 bits per square millimeter.

• Physical density: The diskette physically measures approximately 94 × 90 × 3 millimeters, for a volume of 25,380 cubic millimeters. The physical density by volume is therefore 465 bits per cubic millimeter.

Different storage mediums are compared by density in order to measure the efficiency of the storage. Modern storage densities are quite high: the physical density of a 512 megabyte Secure Digital^® flash memory card is approximately 5,020,000 bits per cubic millimeter (5,400 times that of a "high density" floppy).

Digital magnetic tape is usually measured in "bytes per length"; other media in bits per square millimeter of surface area (since most media, including integrated circuit memory, is still 2-dimensional). However, upcoming media will store data in volumetric space, making volume computations more relevant.

Also, storage densities computed for comparison of technologies usually ignore physical containment; that is, memory density is computed based upon the number of bits relative to the surface of the integrated circuit, rather than the surface of the encasement.

There is a theoretical upper limit to the amount of information that can be stored in a finite space. This limited information density limits the theoretical maximum information capacity of any given object. Practically speaking, technology is many orders of magnitude away from this limit.

See also

• Bit cell — the length, area or volume required to store a single bit

Computer storage

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