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 Figure 8.11). The results are expressed in units of megabits or gigabits per square inch (Mb/sq. inch or Gb/sq. inch) and are used as a measure of efficiency in drive recording technology. Current high-capacity drives record at areal densities exceeding 400Gb/sq. inch.