Can somebody explain why the numbers don't match up? Why is the reported size of the selected items less than subtracting the volume's free space from its total size?

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The free space and total size shown on a volume icon in Computer is accurate. However, selecting all items in the volume's root only shows the total size of items to which you have NTFS Read permissions. This is true even if you have enabled Show hidden files and folders.

Common examples of folders you can't access by default:

  • C:\System Volume Information
  • C:\Users\<another_user's_profile>

Even if you're logged in as an Administrator, by default you do not have permission to access these folders. Therefore the items in these folders are excluded from the total size of the selected items reported in the Properties dialog box.

Tools that use the SYSTEM account to calculate the total size of items are useful for working around this. My favorite is WinDirStat which is a free, lightweight utility that not only can show the total size of items on your disk but can show you which folders are consuming the most space.

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