Why does the space used by files on my hard drive show different numbers depending on how I look?

First let's look at the two examples mentioned in the question. Here's Windows Explorer's view on my C: drive, by right-clicking on the drive and clicking on Properties:

As you can see, Explorer reports that about 47.8 gigabytes are in use, and that 24.6 gigabytes is available as free space. If you're just wondering about the free space on the drive, or how much of the drive is in use, these are actually the numbers I'd use - they take pretty much everything into account.

Now, back in Windows Explorer's view of my C: drive:

Click on any file at the root of C:, and type CTRL+A to select All files. Now, right click on any of the files selected and click on Properties:

You can see that Windows reports the size of all files in C:\. It reports both size, and something called "size on disk". And neither are the close to the 47.8 gig reported to be in use earlier.

Let's clear up "size on disk" first. Windows allocates space for files in "clusters" or "allocation units". The size of a cluster varies, but ranges from 512 bytes to 32K or more. On my C: drive the allocation unit is 4096 bytes. What this means is that Windows will allocate 4096 bytes for any file or portion of a file that is from 1 to 4096 bytes in length. A 1 byte file? That takes up 4096 bytes "on disk". 100 byte file? Still takes up 4096 bytes on disk. A 4097 byte file? That takes up 8192 bytes - one complete 4096 byte cluster, plus another 4096 byte cluster to hold the extra byte.

So in determining how much disk space is used by the files, we need to pay attention to the larger "Size on disk" number.

But that number is still very different than the 47.8 gig.

There are a number of issues that contribute to the difference:

• Hidden and System Files - The Windows Explorer display above was using the default settings which do not display files marked as "hidden" or "system", and as a result they are not included in the calculation. Changing that setting increases the "Size on disk" by approximately 7 gigabytes as all the hidden files on my machine were included in the calculation. Most notably that included the system swap file and hibernation file, both of which are large, and marked as hidden.

• Directories - directories, or the lists and indexes of the files on your machine, are not files themselves, but they do take up space.

• The File Allocation Table or Master File Table - The FAT itself, of FAT file system fame, is actually pretty small. The MFT however, used in NTFS file systems, is also stored on disk but not visible as a file. It defines where the used and free space on a disk actually is. Depending on the size of your disk, your files, and the size of the allocation unit, the MFT can be quite large.

• The Log File - it's a file, but again, you won't see it. This is internal NTFS data.

There's probably much more that I'm not aware of that enters into the picture as well. I know in researching this issue there was no way I could actually get all the numbers to add up. Possible additional culprits further confusing the situation include compressed files (which appear bigger than they really are on disk), junctions and shortcuts which may or may not be counted as files, utilities which may or may not include the recycle bin as part of the "spaced used" since the contents have technically been deleted, and Windows Vista's ability to keep additional versions of files automatically (though where they're kept and how they're managed, I'm currently not sure).

So yes, it is confusing.

But as I said earlier, I would rely on the drive properties to tell you about the actual used/unused state of your drive.