Oracle: Measure disk space used

You need to know how much space your schema is using:

 -- estimated bytes per table used:  
 select   
   table_name,   
   avg_row_len*num_rows/1024/1024 size_mbytes   
 from user_tables   
 order by 2 desc; 

 --top table-segments for bytes used:  
 select   
      owner,   
      segment_name,   
      partition_name,   
      segment_type,   
      round(bytes/1024/1024) size_MB   
 from dba_segments   
 where owner like '%MY_FAVOURITE_NAME%'  
 order by bytes desc;  

 --top table-segments for bytes used: (per Segment_name)  
 select   
      owner,   
      segment_name,   
      sum(size_MB) size_MB   
 from (  
  select   
       owner,   
       segment_name,   
       partition_name,   
       segment_type,   
       round(bytes/1024/1024) size_MB   
  from   
       dba_segments   
 )  
 where owner like '%MY_FAVOURITE_NAME%'  
 group by owner, segment_name  
 order by size_MB desc;

 -- top owner for bytes used:  
 select   
      owner,   
      round(sum(bytes/1024/1024)) size_MB   
 from dba_segments   
 where owner like '%MY_FAVOURITE_NAME%'  
 group by owner   
 order by 2 desc;   

-- disk free  
 select   
      tablespace_name,   
      round(sum(bytes)/(1024*1024*1024),0) GB_free   
 from dba_free_space   
 group by tablespace_name   
 order by 2 desc;