Source:
#! /usr/bin/awk -f

BEGIN {totall=0; totuse=0}

$1 ~ /\/dev\// {totall=totall+$2; totuse=totuse+$3};

END {printf ("\n Total space: %10d \n  Total used: %10d\n", totall, totuse);
     percentage=((totuse*100)/totall);
     print "Percent used:      " percentage "%\n"}


Command:
$ df | dfpercent.awk

Example Output:
 Total space:  303813577 
  Total used:  159883625
Percent used:      52.6256%