du Linux Commands: Estimate file space usage, reports the amount of disk space used by the specified files and for each subdirectory

du -s /dir/

or

du -ah /path/to/dir

-a : all files
-h : human-readable form

du -sb dir (size in Bytes)
du -sk dir (size in KBytes)
du -sm dir (size in MegaBytes)

-s (sumarize , quite mode )