Areal Density, sometimes referred to as “bit density”, is a calculation (the “product”), and the unit of measurement, of the bit density (bits per (square) inch (BPI)), defining how many bits can be written onto one linear inch of a track) multiplied by the track density (tracks per (square) inch (TPI)), defining how many tracks are recorded onto one linear inch of a track), which results in a figure indicating how many bits per square inch are present on the hard disk drives surface. In other words, it is the product of BPI and TPI, which reflects the amount of user data that can be stored reliably in one square unit of area on the surface of a hard disk drive’s platter.  It is now measured in gigabits per square inch (Gb/in2) and is the metric used to quantify the impressive growth in hard disk drive data storage capacity.

 Data is arranged on a hard disk drive in concentric circles, called tracks.  To gain more data on a track, the spacing between each bit in the down-track direction must decrease.  The data density in this direction, also called the linear or recording density, is measured in thousands of bits per inch (KBPI).  Similarly, the track density across the hard disk drive is measured in thousands of tracks per inch (KTPI) – how close-fitting the concentric tracks are per square inch.  The TPI metric not only reflects the width of the written track, but also the small guardbands that are needed between tracks to provide margins for head-to-track misalignment.
 Areal density has increased by almost 8 orders-of-magnitude (10^8) since the introduction of the first hard disk drive.  For decades, the compound annual growth rate (CAGR) of areal density was about 30%.  With the introduction of MR (magneto-resistance) by head technology around 1990-1991, the rate increased to 60%.  When GMR (giant magnet-resistance) heads were introduced by IBM in the late 90's, the CAGR temporarily increased to over 100%, during which time there was an increase in the number of companies that exited the industry or merged.  The pace of areal density growth is now slowing and should settle somewhere between the historical rates of 30 and 60%. Technological advances, although extremely challenging, to the media layer on the platters, actuators, control electronics and other components, especially in the case of read/write heads, are made to continually improve areal density.
 The track-to-track spacing (track pitch) is 8 micro-inches (about 200 nano-meters). The 5–10% guardbands between tracks are a fraction of a micro-inch (less than 20 nano-meters).
 Another important aspect pertaining to areal density is the following. The gradual degradation of magnetic information is often referred to as thermal decay. There may be millions of atoms that are magnetically oriented in a certain direction in one bit. Over time, thermal energy, i.e. heat, will make some of the atoms “forget” their magnetic orientation in the bit. As there are fewer atoms maintaining the data, it follows that it takes even less thermal energy to get a few more atoms to forget the data. After some period of time an “avalanche point” is reached resulting in a random orientation for the bit and the data is lost – it has decayed. This is referred to as the super-paramagnetic effect. The thermal stability of bits drops rapidly as the areal density increases – there are fewer atoms in each bit to retain the magnetic orientation. High temperature environments can make thermal decay worse. It has also been demonstrated that writing to a particular track can cause degradation in the bits in the adjacent tracks. It is not widely known, but many modern hard disk drives routinely check for thermal decay of bits in the field and rewrite the sectors in which degradation is identified.
 See: Data, and Data Loss & Data Recovery.