Areal density is typically expressed in gigabits per square inch (Gb/in 2), or sometimes in terabits per square inch (Tb/in 2) for higher-capacity hard drives. What is the formula for areal density? Areal density is calculated by multiplying the number of bits per inch (BPI) by the number of tracks per inch (TPI).

Areal density is often used as a technology growth-rate indicator for the hard disk drive industry. Areal density is defined as the product of the linear bits per inch (BPI), measured along the length of the tracks around the disk, multiplied by the number of tracks per inch (TPI), measured radially on the disk (see the first image below).

Areal density describes the amount of data that can be stored within a given amount of physical space on a hard drive’s platters. Generally speaking, a higher areal density is a good thing.