Why You Have Less Space on Your Hard Drive Than You Thought

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It’s a common question, “Why do I have less space on my hard drive than I thought?” Now that multi-terabyte drives are common, this question may not seem important to some but the increased use of SSDs and partitioned drives makes it very relevant. Here are some answers.

Different ways of measuring disk size

To begin with, you may start out with the perception of less space than you thought because of the difference between the way that manufacturers label disk sizes and the way that computers measure size. It has to do with decimal-based numbers versus binary-based numbers. Disk manufacturers use the system 1 kilobyte=1000 bytes. Computer systems use 1 kilobyte=1024 bytes. The discrepancy between the two systems grows with disk size. A 500 GB drive will be read by the operating system to have only about 466 GB. Of course, the bytes haven’t gone anywhere. A byte is still a byte but the different ways of measuring size can cause confusion.

Perceptions aside, there are many real ways that disk space gets used up  even when you don’t put any files of your own on a disk.

NTFS file system overhead

First of all, Windows systems likely have the file system NTFS and this file system comes with overhead. It stores file information in something called the Master File Table (MFT), which in its default configuration can occupy as much as 12.5% of free disk space. The amount that MFT actually uses varies according to your system and usage patterns.  Examples of MFT sizes that I have seen range from a few hundred MB to several GB.

Hidden partitions

In newer systems with Windows 7 or 8, there is likely a hidden partition called the System Reserved Partition. It has no letter and is there to support BitLocker encryption, the boot configuration database, and the Windows Recovery Environment. In Windows 7 it is 100 MB and in Windows 8 it is 350 MB. You can read more about it at this link.

Most PCs also come with a hidden recovery partition that holds a Windows image to be used as a backup for reinstalling the original OEM setup. This partition is typically upwards of 10 GB or more.

Windows system files

Then there is the operating system - about 10 GB for Windows 7 or 8.  But along with the operating system itself, there are a number of accompanying space consumers. Here are some hidden system files to look out for.

  • There will be a page file whose size usually is 1 to 3 times the size of the installed RAM. It can be adjusted. See this Microsoft reference
  • If you use hibernation, the Hiberfils.sys file may occupy space equal to size of the installed RAM. Managing this file is discussed in this article.
  • If you have upgraded the operating system, there may be a large backup. For example, see this article.
  • System Restore points – Typically, 3 to 5 percent of the disk but at least 300 MB. Can be adjusted, See this Microsoft reference.

It is also worth noting a related situation. It isn't uncommon for a laptop to come with a hard drive partitioned into a C: system volume and a D: data volume with the C: partition being rather small. In this type of setup, there will not be a lot of free space on the C: drive.

And there you have it - some ways your disk space gets used up before you even start.

Related article: Best Free Disk Space Analyzer

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This tips section is maintained by Vic Laurie. Vic runs several websites with Windows how-to's, guides, and tutorials, including a site for learning about Windows and the Internet and another with Windows 7 tips.

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Comments

I use them for a lot of things most notably backing up. I use hard links into my Dropbox which works like a charm (but the stuff is counted twice as far as space used is concerned)

Nice article. I believe it also follows and counts Hard links as their full size even though that is not the case. There is software out there that will give you the real story on what is actually used

What hard links are you referring to? As far as I know, none of the items in the article involves hard links. Windows uses "junctions" for some things but these are not hard links and anyway are not involved in this article's discussion. See http://windows7tips.com/symlinks-definition.html

I use symlinks and hard links on my drives. I HAVE HEARD that when WINDOWS figures the size of used space, it (incorrectly) reads the the hard linked file as actually being there. I may be wrong. I felt it pertinent at the time to mention that as you may have more hard drive space than windows is reporting
As for the tool you posted, 5 stars. I was able to use it to reclaim 16 GB of space by clearing some things of which I had not been aware

You are right that Windows does not always report hard links correctly. Perhaps you are thinking about the WinSxS directory, which does use hard links.