Why it shows less space for memory devices

Since long I have seen on my PC, storage devices always shows less space than its original space. The common answer I got was like this "OS takes that space for some processes and that's why the available space is less". But as time goes by I am sure all of us know the reason. But still just to have the real reason to be available in our knowledge base(DotNetSpider)I thought of explaining it.

Recently I bought a 500GB Western Digital external hard drive but when I plug it in my laptop it shows 465GB available memory space. Where is the rest 35GB?

Our operating system is a program and according to that the memory space is calculated in this way

1024 bytes = 1 KB
1024 KB = 1 MB
1024 MB = 1 GB.

But the manufacturer companies calculate the memory space in the following way

1000 bytes = 1 KB
1000 KB = 1 MB
1000 MB = 1 GB

So when a hard drive company manufactures a hard drive of 500 GB then the space in bytes will be 500*1000*1000*1000 = 500000000000bytes. Now when this hard drive is connected with computer then the space is calculated by the OS. The space conversion will be

500000000000/1024 = 488281250 KB
488281250/1024 = 476837.1582 MB
476837.15820/1024 = 465.66 GB

So though the manufacturer tells the space is 500GB but the OS can only see 465GB as available memory space.

A very general discussion but still it can be helpful for those who don't know this.