du -h your_directory
# gives you the size of your target directory.
du -hcs your_directory
# If you want a brief output.
du -sh /var
# returns the size of all content inside the directory
du -hs /path/to/folder
du -h --max-depth=1 /directory
du -sh *
du -sh /directory
du -shx /directory
du -a / | sort -n -r | head -n 10
5351116 /
2462616 /usr
2153492 /home
2153472 /home/george
1571924 /usr/lib