Today I'm going to answer a question that I get asked far too often:  Why does my hard drive claim to be one size, but my computer tells me that it's much smaller? Here's the incorrect answer that I hear most people say when trying to answer someone about it (trying to seem like a know-it-all?):  It's because Windows (or whatever operating system) needs that space for Windows stuff. Wrong!  If Windows is installed on your hard drive, that's the space that Windows is using (or other OS).  It doesn't need to occupy every hard drive you own. Let's say you just bought a shiny, new hard drive.  On the box, it says 500 GB.  You figure that's just enough to backup your 470 GB worth of movies (you're a movie fiend), with room to spare.  So you leave Best Buy and go home, plug your new hard drive into your computer, and... Oh no!

<geek_explanation alt="skip if you want the short answer">
Hard drive manufacturers calculate sizes using the typical base-10 relationship (1,000 bytes = 1 KB; 1,000 KB = 1 MB; 1,000 MB = 1 GB).  This would work just fine, if the logic in computers was built that way.  Unfortunately, it's not. Computers work using something called a bit.  A bit has only two options: off or on (0 or 1).  So because everything is based off of these bits, all data in a computer is calculated using a base-2 system.  Since we humans are used to base-10 counting, this is a bit unusual for us (no pun intended). So, when a computer says "kilobyte" (KB), it means 2^10 bytes, or 1,024 bytes.  Not 1,000. Therefore, a "megabyte" (MB) is 2^20 bytes, or 1,048,576 bytes, or 1,024 kilobytes.  And a "gigabyte" (GB) is 2^30 bytes, or 1,073,741,824 bytes, or 1,048,576 kilobytes, or 1,024 megabytes.
</geek_explanation>

In essence, hard drive manufacturers decided that they would count a kilobyte as 1,000 bytes, while computers actually count a kilobyte as 1,024 bytes (because of the way computers manipulate data -- see above). So let's take a look at your hard drive again.  It's 500 GB, which to the manufacturer means this:

500,000,000,000 bytes / 1,000 = 500,000,000 kilobytes
500,000,000 kilobytes / 1,000 = 500,000 megabytes
500,000 megabytes / 1,000 = 500 gigabytes
500 gigabytes = "looks like a bigger number to the unknowing customer"

But the computer sees your hard drive like this:
500,000,000,000 bytes / 1,024 = 488,281,250 kilobytes
488,281,250 kilobytes / 1,024 = 476,837 megabytes
476,837 megabytes / 1,024 = 465 gigabytes
465 gigabytes = "35 less gigabytes than you thought you paid for"

So essentially, the manufacturer sold you 465 gigabytes, but told you that you were getting 500 gigabytes.  They simply had a different definition. And if you think getting screwed for 35 gigabytes is bad, just keep in mind that it gets much worse the bigger you go.  My friend recently bought himself a 2 terabyte (that's one order of magnitude greater than a gigabyte, for those that don't know) system, in order to store all his movies and music.  Applying the same calculations, my friend was out almost 200 gigabytes.  That's larger than many peoples' entire hard drives.  In fact, a 200 GB Seagate laptop hard drive from TigerDirect.com (shown here) costs about $80.  Would you be upset if you lost $80*?
*if not, e-mail me and I'll give you my PayPal address... I could use $80 ;)

So why isn't anyone complaining?  Well, some people are complaining, but they won't get much attention from the media.  In fact, Seagate, a very large hard drive manufacturer, was sued in 2005 for this very issue.  Seagate has since agreed to settle (see article).

So the next time you decide to purchase a hard drive from a store, tell them you're only going to pay for the gigabytes that you can use!  Haha, yea right...