I'm about to backup my f-13 system so I can install CentOS instead. I'd like to determine just how much disk space I need to make a backup so I don't have to buy a new external harddrive right now.
To calculate the size just use:
It will return the size in human readable format, it should look like this:
Meaning my current directory would require about 8MB to backup.
Generally, a backup only includes data files.
The way to tell how much space it'll require is to look at your data file folders and add the total sizes up from each. Then give yourself sufficient headroom for future backups, and buy a disk that works.
But if you mean you wish to create an image of one system that you could then use to "go back to" completely from the new OS, then you'll be backing up everything, the entire contents of that OS, and thus you'll need to adjust your size accordingly.
UPDATE response to comment:
Data files as opposed to program files. Program files are files associated with programs and in a home user environment they are not usually backed up.
Data files are files that cannot be replaced simply by reinstalling a program. They contains settings you've set, information you've created or captured. Documents, pictures, music, bookmarks/favorites, etc. They are the files you generally need to keep and thus are the files you back up.
By F-13 I assume you mean Fedora, or linux (Note: If this is the case, please add some tags to your initial question clarifying this). The files you'd backup would be in your home directory or usr directory.
In the second scenario, you're not looking for a backup so much as an image, which is a bit-for-bit copy of the entire hard disk or partition. In this case you'd need a tool to create the image and you'd need a backup harddrive at least as big as the total used size of your current hard drive. I do not recall specifically how to see the total used space of your hard drive in Fedora.
But, with hard drives being as cheap and large as they are, I'd say spend $100-$150 and get a 2 terabyte hard drive. Unless you rip every movie you own and store them on your computer, you're probably not going to run out of space with that.