How to find out size of directory in AIX

NODE1$ du -ak /u01|sort -n -r|head
21140740        /u01
21137164        /u01/oracle
21123992        /u01/oracle/app
10759480        /u01/oracle/app/diag
10230288        /u01/oracle/app/product/11.2.0/db_1
10230288        /u01/oracle/app/product/11.2.0
10230288        /u01/oracle/app/product
9821492 /u01/oracle/app/diag/rdbms/cbsdb/cbsdb
9821492 /u01/oracle/app/diag/rdbms/cbsdb
9821492 /u01/oracle/app/diag/rdbms