With no arguments or options, the df command displays the amount of free disk space on all of the mounted file systems. For each file system, the df command reports the file system's configured size in 512-byte blocks, unless you specify the -k option, which reports the size in kilobyte blocks. The command displays the total amount of space, the amount presently used, the amount presently available (free), the percentage used, and the directory on which the file system is mounted.
source:
http://www.helsinki.fi/atk/unix/dec_...U_012.HTM#I847
thank u, Finnland
