Tuesday, September 13, 2005

I thought I bought a 120 GB Drive, but the system reports 111 GB.

Where did the other 9 GB go?

I've seen this question a few times on various newsgroups and support forums. Basically, here's the situation:

For whatever reason, drive manufacturers long ago began reporting their drive sizes in thousands of bytes and labeling that KB, millions of bytes and labeling that MB, and billions of bytes and labeling that GB (and one day trillions of bytes labeled TB). On the surface this seems to make sense; after all, 1000 meters is a Kilometer. So why does the system report that a drive labeled by the vendor as 120 GB is actually 111 GB? This is because the world of computers does not fit nicely into the efficiencies of the metric system, even though drive manufacturers would like us to think so. In the metric system, 1000 units of something is equal to 1 Kilo-unit, 1 million equals 1 Mega-unit, and so on. However, bytes are not a true metric unit, and 1 Kilobyte is actually equal to 1024 bytes, NOT 1000 bytes. It follows that 1 MB is equal to 1024 KB, 1 GB is equal to 1024 MB, and so on. In Windows, drive size is reported using the true number of bytes on the drive represented as GB (or MB for small drives), not millions or billions of bytes like drive manufacturers use.

Using our old high school chemistry method of unit conversion, we can see that this is true for my 120 "GB" drive:

120,000,000,000 bytes * (1 KB/1024 bytes) * (1 MB/1024 KB) * (1 GB/1024 MB) = ???

We can see that bytes, KB, and MB cancel out, and were left with (120,000,000,000 GB)/(1024*1024*1024) which equals 111.76 GB. Looking at my computer, or right clicking on my 120 GB drive and looking at the properties reveals that Windows reports the drive size as 111 GB, which is the true size of the drive.

So there is no missing 9 GB, it is simply a difference in how drive manufacturers label their drive and how Windows calculates and reports drive size.

- Tony

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