How does a computer calculate used space?

On my computer, an HP-Pavilion, if I click on the my hard drive, it shows that I have used up about 11.4 GB of space. If I look at all the files on the drive and add up all their sizes, I get something around 5 GB. What is taking up the rest of the space and where is it?

Hidden files, swap files etc… BTW how did you add up all the files on your computer? By hand? There’s probably over 15000 files on your HD, that seems kinda time consuming. Anyways, I’d imagine, the FAT knows exactly how much of the space is used/free, since that has alot to do with it’s main job of reading/writing files.

I clicked on the properties of the folders. There were only about 10 and most of the data was stored in only three of them.

10 files? On your computer???

While I don’t know why you have a big discrepancy, disk space is calculated by a big table per filesystem that holds pointers to blocks of data. It simply counts up the number used and multiplies by the block size to get free space.

You might want to do some sort of disk repair prorgam to make sure everything is in order. There is one included in some version of Windows. right-click your drive and go to the “Tools” tab (or something like that, sorry I’m not on Windows now).

Oh, on second thought, you would expect some discrepancy because the total size is probably (see above) calculated from block size, while looking at the sizes of files gives you the size of the files. If your block size is 4k, and your file is 1 byte, you have taken up 4095 bytes on your hard disk. If that’s all thats on your hard drive, you would see almost 4k used, but just 1 byte in total file size.

So perhaps you have a lot of small files?

"Slack Space -The amount of disk space that is wasted by having a large cluster size. For example, if a 300-byte file is stored on a disk with a cluster size of 1,024 bytes, there will be 724 bytes of slack space that can’t be used for any other files. You can see how much space is allocated to a file by typing “DIR /v” at the command prompt. "

Taken from

But almost 6.5 gigs of slack space?! That’s a bit overboard isn’t it?

There are hidden folders which are taking up most of your space. System Restore is a likely culprit (I had to completely disable System Restore on my computer because it ate up every last MB of space, despite my trying to limit it size). To see these hidden folders, you must first go to Tools>folder options>view and select to show hidden files and folders.


Thank you very much! The restore file contains 6.5 GB. I knew I came to the right place for an answer!

Mad Scientist

Now the question is can I just delete that folder without harm to my system?


I wouldn’t delete the System Restore folder, as I would imagine that might cause other problems.

You should be able to just restrict the space used to something more sensible. I think you do this by right clicking ‘my computer’, going on the performance tab, and clicking file system, I think the option to reduce the space is on the hard disk tab there, which should be the first page shown. Then you can reduce the space used to something sensible, or disable System Restore all together if you want.

I had this when I got my current computer, and the default was 10% of harddisk space, which means my 20 Gig hard drive had abour 2 gig of it used by system restore. I now have this set to 200 Meg, which seems more sensible.

I found that in the restore folder is an archive folder is taking up just about all the space in the restore folder. It looks like the computer is making updates about 2-3 times a day. The files in the archive go back 2 months. Can I clear some of those files out and recover the space? I have reset the system restore size to the minimum of 200 MB. On my computer the max was suppose to be about 5GB but there is more then 6 GB of data in the archive file.


I would just wait, and see how much space you have, if you’ve updated the amount of space used. I gather that restore saves things more often the more disk space you give it, and eliminates old backups. If this is the case, then as it eliminates old files it doesn’t make as many new ones, you should gain space over time. It might be worth monitoring this, to see if this is the case.

However, there may be a way to eliminate old files before then, but I don’t know how at the moment, as I don’t have access to a PC with windows Me, although I will have tommorow, and I can get back to you on monday, when I can next get on the internet.

I wouldn’t recommend just deleting the files manually, as I don’t know how well system restore would cope with that sort of thing.

I was mistaken that the files only go back 2 months. They go back to Sep 2001.

I found out how to delete those files. The files in the Archive folder were the system restore points. Since I never cleared those files out before, they went back to September of 2001 when i purchased my computer. This cleared out that extra 6 GB of space.

To remove these system restore points, go to control panel and click on system. Select the performance tab and pick file system. Select the troubleshooting tab and put a check in the disable system restore box. Click Apply. You will need to reboot the system for the changes to take place. After the reboot, all the files will be cleared and the hardrive space will now be free.

Thanks for all your help


If your running winME, there is a bug that means that system restore will ignore the maximum sze limt and expand until it fills your drive. I think there may be a patch on the MS website.

If I read this correctly, you are now doing your trapeze act without a net. While you certainly don’t want the system restore points chewing up all your available hard drive space, you really don’t want to operate without them at all.

I recently let MS’s WindowsUpdate site download an upgraded Intel video driver that rendered my machine totally unusable (imagine a screen full of random blocks of color - tough to click on something you can’t see!). I managed to bring it up in Safe Mode, and retreated to the last system restore point. Without that capability, I’d have had a hell of a time trying to reload the old video driver. There are times when the system restore points are your friends!

That’s not that tough. First, set the video driver to standard VGA. Reboot into Windows. Find old driver and install. Viola!

System Restore is, IMO, overhyped. While it may be useful for people who have no clue or no desire to troubleshoot, for experienced users it can end up being a waste of hard disk space, especially the WinME implementation.