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There is only one partition for both system files and personal data, which is C:. The other two partitions are for boot and for recovery.

Now I have two different reports on the size of C: on my Windows 7:

  1. by clicking the property of C:, it shows 59.4 GB for "used space";
  2. by selecting all contents (I have setup to view all hidden files and directories) under C: and viewing their property, it shows 46.9 GB for "size" and 47.6 GB for "size on disk".

Here are my questions:

  1. I was wondering why the two reports are not the same, and where the difference comes?
  2. in the second way, why "size" and "size on disk" are different? What do they mean actually? What is the difference between them and "used space" in the first way?

Thanks in advance!

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5 Answers 5

up vote 6 down vote accepted
  1. There's some files and folders that you don't have access to, even as Administrator. System Volume Information is one such folder.
  2. You can't directly see some items that take up space, like the MFT and USN Journal.
  3. On Vista & 7, the Windows directory is densely hardlinked. Explorer is unable to handle this and will count files with n hardlinks n times.
  4. Explorer doesn't take NTFS Alternate Data Streams into account.
  5. Files locked for exclusive use won't be counted either.

**No matter what you do, you're not going to be able to select all the files/folders on the drive and make the number match the drive's properties.

As for "size" vs "size on disk", there's several things that make the two differ. "Slack space", as mentioned by the others are the most common. But files stored with NTFS compression may take up less space on disk, affecting Explorer's display. Sparse files, offline files, and other special attributes can also cause the 2 to differ significantly.

**You can use a linux based os or a live cd to see all the hidden folders. For Windows 7 one of those folders is called ProgramData and it accounts for the big difference in size reports.

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There is one more thing - if there are 1000 files with 5 byte size and your filesystem has 1024 byte cluster size - these files will use 1000 clusters and used space will be 1024 x 1000 bytes, not 5 x 1000 bytes. –  Kamil Nov 22 at 7:53
    
@Kamil: That difference is called slack space, and already addressed by several answers. –  afrazier Nov 22 at 14:28
  1. Are you sure you selected everything? No hidden files in the root of C:\ that you missed? e.g. your page file or hibernate file?
  2. Size refers to the sum of the total bytes in the file whereas size on disk refers to how much space they are taking up on the disk. File sizes are aligned to multiples of 4K boundaries (it's faster), so a 1 byte file still takes up 4K on the disk.
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Thanks! 1. I have setup to view all hidden files and directories. Are page file, hibernate file and possibly others therefore viewable? If not, how to select them? 2. So "size on disk" and "used space" are same? –  Tim Feb 4 '11 at 2:56
    
Size on disk and used space will never match as explained by both Mikel and Rhys –  Moab Feb 4 '11 at 3:05
    
@Moab: there are three terms here: size, size on disk and used space. both Mikel and Rhys explained size and size on disk are different. But in comment I asked if size on disk and used space are same. –  Tim Feb 4 '11 at 3:07
    
Size on disk and used space should be the same as far as I am aware. –  Rhys Gibson Feb 4 '11 at 4:19
    
Size on disk will also vary if you've enabled compression on any files as size will show the actual size of the file and size on disk will show the compressed size –  Matthew Steeples Feb 4 '11 at 11:02

For the totals to come close, you also need to uncheck "Hide protected operating system files (Recommended)".

Get there from Windows Explorer by pressing Alt+T, then clicking on Folder Options, then the View tab.

Otherwise, selecting all files is not selecting pagefile.sys, which is typically about 4 GB.


Size and size on disk are different because of how hard drives and file systems work.

Every file uses a certain number of blocks. Typically a block would be about 4 KB. So when you save a file that's 1 KB, it actually uses up 4 KB of disk space. A 5 KB file uses 8 KB, etc.


Update

Found a thread on MSDN called NTFS Misreports Free Space?.

It suggests running chkdsk to see a more detailed breakdown. Note that you must run it as Administrator.

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Thanks! After unchecking that, selecting all contents and clicking property, it shows 50.3 GB for size and 50.9 GB for size on disk. Both are still smaller than 59.3 GB got by clicking property of C:. Do you know why? –  Tim Feb 4 '11 at 3:03
    
Not sure. Perhaps system restore reserves some? –  Mikel Feb 4 '11 at 3:11
    
Do you happen to know where the system restored files are? –  Tim Feb 4 '11 at 3:14
    
System Restore files are under the System Volume Information folder –  Rhys Gibson Feb 4 '11 at 4:19

One other item you're missing is that you don't have permissions to see inside folders for others users by default, even as administrator. Even if you're the only real user of the system, there's likely a few other folders in the c:\users folder that belong to system accounts of various persuasions.

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Are you referring to the difference in the size when you look at disk properties? - For example, my Hard Disk shows my Used Space at 47,575,797,760 bytes (44.3 GB).

The reason for this is because 1 GB is not actually 1 billion bytes, it's 2^30 bytes. It's all built to support binary, so 1 KB is actually 2^10 bytes, 1 MB is 2^20 bytes, and 1 GB is 2^30 bytes --> which is why 1 GB RAM = 2^10 MB, which is 1024 MB.

So in my case, 47,575,797,760 bytes = (47,575,797,760)/(2^30) = 44.3084144..., which rounds to 44.3 GB, as indicated.

It can be confusing because we recognize, by common definition that Giga means 10^9, but for binary systems, Giga means 2^30.

So, when you buy a "100 GB HD", what you're actually buying is 100 billion bytes, which is recognized by the computer as 93.1 GB, as (100,000,000,000)/(2^30) ~ 93.1 GB.

There's nothing wrong and it's not because of hidden files or different size clusters.

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