Originally Posted by Mr. Alex
Gnome-system-monitor. "File systems" tab. There are free and available columns. And available is always less then free? Why?
Free space is space that's not used to store any meaningful information.
Available space is space you can use to store your files.
The difference between both numbers can be because of many things. Your user might have a disk quota assigned, and some file systems reserve some disk space, so no one can use it. This is done to guarantee that the root user will have some available space for diagnosis and administrative purposes if needed. You can set this reserved space when formatting the volume or at any time using tune2fs.
However that's an fs specific feature. ext2/3/4 work this way. Other fs's might not have this feature, or it might work in a different way.