The term ”form factor” is used in the computer industry to describe the shape and size of computer components, like drives, motherboards and power supplies. When hard disk drives initially made their way into microprocessor-based computers, they used magnetic platters up to 8 inches in diameter. Because that was the largest single component inside the HDD, it defined the minimum width of the HDD housing—the metal box around the guts of the drive.

The height was dictated by the number of platters stacked on the motor (about 14 for the largest configurations). Over time the standard size of the magnetic patter diameter shrank, which allowed the HDD width to decrease as well. The computer industry used the platter diameter dimensions to describe the HDD form factors, and those contours shrank over the years. Those 8” HDDs for datacenter storage and desktop PCs shed size to 5” to today’s 3.5”, and laptop HDDs, starting at 2.5”, are now as small as 1.8”.