What Is a 3.5 inch Motherboard?

Published on:
August 22, 2019
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Most tech consumers are familiar with the ATX, Micro ATX and even Mini ITX form factors that proliferate the computer market. But once you step off the cozy platform of consumer computers and into the world of Industrial and OEM computers, you are bound to be hit by form factors and pieces of tech that you’ve had no familiarity with beforehand. The 3.5 inch motherboard is one such form factor.

The 3.5 Inch motherboard is a common industrial motherboard form factor with size specifications that are approximately the same dimensions of a 3.5 inch hard drive, which it is named after. It has a small size, but not so small that it cannot be made to be I/O rich through intelligent design, which makes it a perfect fit for a wide range of industrial applications.

Contrary to its name, however, it is not actually 3.5 inches in width or length. Its actual dimensions are 5.75” x 4” (146mm x 101.6mm). This can be quite confusing at first for those unfamiliar with the form factor, but the important thing to remember is that it is the same size as a 3.5 inch hard drive. And in case you didn’t know, 3.5 inch hard drives aren’t 3.5 inches wide either. That makes it a pretty misleading name for the uninitiated.


Why is it called a 3.5 inch motherboard if it’s not 3.5 inches?

So the important thing to take from this is that names in the world of computers don’t make sense, and it’s not worthwhile to even try, right? …Well not exactly. The name actually does make sense, in a sort of convoluted historical sort of way.

As mentioned earlier, the 3.5 inch motherboard form factor is named after the hard drive of the same name. But why is the hard drive described as 3.5 inches when it’s actually 4 inches wide? The answer, “naturally”, goes back to that archaic little piece of storage technology called the Floppy Disk. 

For those younger readers unfamiliar with the floppy disk, and not to make anyone feel dated, it was an old, mostly obsolete piece of storage technology that functioned by storing information on a thin, flexible magnetic disk. Because this disk was thin and flexible, it was called a floppy disk. As you may have guessed, this disk was 3.5 inches in diameter. BUT, because the thin magnetic disk was so fragile, it was placed in a square, hard plastic case. This case was… you guessed it. 4 inches wide. Hence why the 3.5 inch hard drive has its name and hence why the 3.5 inch motherboard is actually 4 inches wide. Pretty neat.