Hard drive sizes are not the same as disk space

by on March 25, 2010

Q: I have a hard drive that claims to be 750GB but when I go into Windows on my computer and look at the disk size it says that it is only about 700GB. How come I am missing 50GB of disk space and where did it go?

A: What you are seeing is normal because operating system makers and hard drive manufacturers define a Gigabyte differently. Hard drive manufacturers define a Gigabyte as 1000 Megabytes. This probably seem logical to most people but in the computer world everything is based on a power of 2 so we define a Gigabyte as 1024 Megabytes and this means what the hard drive manufacturers say is 1 MB the operating systems say is a little less than 1 MB. Once you get into hard drives that are hundreds of Gigs then the difference really starts to add up as you see by your missing 50 GB.

Why do geeks count by a power of 2? First lets discuss what we do in the real world. In the real world we count by 10s. The problem here is that to make a computer count by 10 is very difficult, at least when computers were being created, because computers run on electricity. I like to equate counting by 2 to a light switch and counting by 10 to a dimmer switch. It is a lot easier to know if the light is on or off with a light switch than it is to know if the light is at a 4 or a 5 with a dimmer switch.

So way back when computers were created the computer engineers came up with the idea to count by a 2’s (on or off) to make everything in the computers work easier. When you count by 2’s you don’t get perfectly round numbers like you do when you count by 10’s so our even number (for lack of a better term) is 1024 when counting by 2’s vs 1000 when counting by 10’s.

So hopefully that gives you a little better understanding of why the advertised hard drive space, which if you read closely will specifically say on the box that it is based on 1000 MB, is different from what your computer says you have for hard drive space.

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