Why Does My Hard Drive Show Less Space Than Advertised?

First, we need to explain how computer storage size is measured...

Computer file sizes are measured in bytes, kilobytes (kB), megabytes (MB), gigabytes (GB) & Terabytes (TB). And there are higher values too, which we'll come to later.

One byte is equal to 8 bits - where a 'bit' is a 'binary digit' which can be just 1 or 0. In binary (which is a base 2 numbering system - as opposed to our usual 'base 10' or 'decimal' system), eight bits can therefore represent integer (whole) numbers from 0 to 256.

When we count in decimal, each time we hit the number 9, we zero that column and add one to the column the left, so 9 becomes 10, and 99 becomes 100 etc. In binary, the same thing is happening, except we only have the digits 0 and 1, and not 0-9. So all numbers can be expressed as 1's and 0's - or on/off signals.

See Wikipedia's 'Counting in Binary' for a full explanation :


The easiest way to imagine a byte is as one character of text, although file sizes will not just be made up of the data they store. There will be a lot of additional information stored with differing file-types. And different encoding systems can use more than 1 byte to represent one character of text; but let's keep it simple for now.

  • 1000 bytes is equal to 1 kilobyte (kB). A one page word processing document with no images would probably be 5-10kB.
  • 1024 bytes (which is the closest 'round' number in binary - or 10000000000 or 2^10) is equal to 1KB (uppercase 'K').
  • 1000 kB is equal to 1 megabyte (MB). Typically 1 MB would be equivalent to a medium-sized paperback book in terms of data size. 700 MB is the standard size of a music or data CD.
  • 1000 MB is equal to 1 gigibyte (GB). 4.3 GB is the approximate size of a standard single-layer, single-sided DVD. Blu-Ray discs store approximately 25GB on a single layer or side.
  • 1000 GB is equal to 1 terabyte (TB). Current computer hard drives typically offer 1 to 4 TB of total storage capacity.
  • 1000 TB is equal to 1 petabyte (PB). Google processes over 20 PB of information per day... That’s a LOT of data!

So why does a hard drive seem to have less storage space than it's supposed to?

The 'issue' is caused by the discrepancy in binary counting as compared to reporting in whole thousands. i.e. the difference between 1,000 (a 'human' thousand) and 1,024 (a 'computer' thousand).


A 1 TB (1,000 GB) hard drive will only seem to have around 930 GB free for your use. But this is entirely due to differences in counting notation.

Hard Drive manufacturers prefer the decimal system (1 MB = 1000 kB), but your computer uses binary bases (1MB = 1024 KB). The bigger the storage capacity, the farther away you get from matching numbers.

At the 1TB level, the difference between systems is around 9% - which explains why a 1TB HD (1,000 GB) is showing as 930GB (1024GB - 9%)!

It's 1000 (GB) x 1000 (MB) x 1000 (kB) x 1000 bytes = 1 Trillion bytes...

1 Trillion ÷ 1024 ÷ 1024 ÷ 1024 = 931GB (according to binary-based reporting).

Confusing, we know, but at the end of the day, just remember that you haven’t lost any space – it’s just being counted/reported differently!

More information : https://en.wikipedia.org/wiki/Kilobyte