Thread: Areal Density

Areal Density
Areal density, also sometimes called bit density, refers to the amount of data that can be stored in a given amount of hard disk platter "real estate". Since disk platters surfaces are of course two-dimensional, areal density is a measure of the number of bits that can be stored in unit area. It is usually expressed in bits per square inch (BPS\). Being a two-dimensional measure, areal density is computed as the product of two other one-dimensional density measures namely Track Density and Recording Density:



Track Density: This is a measure of how tightly the concentric tracks on the disk are packed; how many tracks can be placed down in an inch of radius on the platters. For example, if we have a platter that is 3.74" in diameter, that's about 1.87 inches. Of course the inner portion of the platter is where the spindle is, and the very outside of the platter can't be used either. Let's say about 1.2 inches of length along the radius is usable for storage. If in that amount of space the hard disk has 22,000 tracks, the track density of the drive would be approximately 18,333 tracks per inch (TPI).

Unear or Recording Density: This is a measure of how tightly the bits are packed within a length of track. If in a given inch of a track we can record 200,000 bits of information. then the linear density for that track is 200,000 bits per inch per track (BPI). Every track on the surtaceof a platter is a different length (because they are concentric circles). and not every track is written with the same qensity.