Schema Space Consumption

Set Pagesize 100
Column GB    Format 99.99
Column Owner Format A30
Break on Report
Compute Sum of GB on Report

Select        RPad(Owner,30,' .')
            As
       Owner, 
       -------------------------------------------------------
              Round ( Sum(Sum_Bytes) / 1024 / 1024 / 1024, 2 ) 
            As
       Gb
  From
       --+----------------------------------------------------
       (
         Select Owner, 
                --------------------
                          Sum(Bytes) 
                          As
                Sum_Bytes
                --------------------
           From Dba_Segments ds
          Where Exists (
                        Select Username
                          From Dba_Users du
                         Where du.Username = ds.Owner
                       )
          Group By Owner
                 , Segment_Name
                 , Segment_Type
       )
       --+----------------------------------------------------
 Group By Owner
 Order By GB Desc;