What is areal density?

Areal density refers to the number of bits that can be accommodated on a length of track or per square inch of any storage surface. In disk drives, the number of bits per inch, or bpi, multiplied by the number of tracks per inch, or tpi, gives the areal density.

IBM was the company that introduced the RAMAC in 1956, which is the first hard disk computer. RAMAC's areal density was only two thousand bits per square inch. This has increased dramatically since then. As of 2014, disks have reached a capacity of 100 billion bits, or 100 gigabits, per square inch.