Why do Hard Drives have less capacity than advertised?

Pile of Hard Drives

My friend recently bought a 1TB external hard drive and complained that he got ripped off. Upon questioning him, I found out that his 1TB hard drive had only 920-ish GB.

The first thing I did was check if there was anything on the hard drive and found some bundled software, tutorial videos and user manuals in all imaginable languages on the drive. I went back to My Computer, right clicked the drive and formatted it.

After formatting the drive we still end up with less than 930GB of available space which did seem like a rip off as more than 70GB or 7% of the capacity was missing.

I went ahead and tried to find the reason for this loss of capacity and found that there was a difference in the way hard drive capacity was calculated by the manufacturers and the computer.

While the manufacturer counts 1TB as 1000GB, 1GB as 1000MB and 1MB as 1000kB, computers do it different, for them 1MB is 1024kB, 1GB is 1024MB and 1TB is 1024GB.

So here’s the maths

1000 x 1000 x 1000 x 1000 = 1,000,000,000,000kB

(((1,000,000,000,000  / 1024) / 1024) / 1024) = 931.32 from which around 69GB seems to be missing.

This clearly explains the missing capacity and thus answers all of who have noticed this difference in capacity.


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