The number of bits per square inch of storage surface. It typically refers to magnetic disk and tape, where the number of bits per inch (bpi) times the number of tracks per inch (tpi) yields the areal density.
Magnetic Disk - From 2K to 1T and Beyond
The areal density of disk storage has increased dramatically since IBM introduced the RAMAC hard disk computer in 1956. RAMAC had an areal density of two thousand bits per square inch, which must have seemed quite tiny in those days. Current-day disks have reached and exceeded one terabit per square inch. In 2021, Western Digital's 20TB drive reached an areal density of 1.16 terabits per square inch.
Magnetic Tape - From 6.7 to 201 Gb
Magnetic tape density increased by a factor of 30 in slightly more than a decade. In 2006, IBM debuted magnetic tape with a bit density of 6.7 gigabits per square inch (GbSI). In 2017, IBM and Sony introduced "sputtered media" tape with a density of 201 GbSI, resulting in a tape cartrdige that holds more than 300 terabytes. The sputtered media means that the magnetic particles on the tape are a fraction of the size of previous particles. See
perpendicular recording,
superparamagnetic limit,
holographic storage and
AFM.