Hard disk drives store their sequence of data bits in linear tracks, which follow around a magnetic disk in concentric circles. In order to fit more data over a smaller area, the gaps between the data tracks must reduce, as well as the spacing between the bits. These dimension quantities are measured in bits per inch (in a circumference-wise direction), and tracks per inch (in a radius-wise direction). These get smaller as the technology gets better; also, one must take into account the tiny areas or gaps between tracks that are there to compensate for a slightly misaligned reader head, so it doesn’t read the neighboring tracks.
The method of measuring the total quantity of data storable over a surface area is quantity per square inch, these days, gigabytes per square inch. This measurement is a combination of your bits per inch and tracks per inch. It is known as the areal density. This density has, since that first wardrobe hard drive, had a steady increase at 30 percent per year, and from the early nineties when magneto head resistance was brought about, jumped to 60. Then again in the late nineties, it leaped up to 100 percent yearly increase with the introduction of giant magnet resistance head technology. Now though, the rate of growth has slowed to under 60 percent.