How to calculate or track monthly database growth?
below query using DBA_HIST_SEG_STAT and base on awr SNAPSHOTS and their priod gives the database growth per snapshot. you can change it for monthly output.

    set serveroutput on     
    execute dbms_output.enable(buffer_size =&gt; NULL);**


    Declare
          v_BaselineSize  number(20);
          v_CurrentSize   number(20);
          v_TotalGrowth   number(20);
          v_Space     number(20);
          cursor usageHist is
                  select a.snap_id,
                  SNAP_TIME,
                  sum(TOTAL_SPACE_ALLOCATED_DELTA) over ( order by a.SNAP_ID) ProgSum
              from
                  (select SNAP_ID,
                      sum(SPACE_ALLOCATED_DELTA) TOTAL_SPACE_ALLOCATED_DELTA
                  from DBA_HIST_SEG_STAT
                  group by SNAP_ID
                  having sum(SPACE_ALLOCATED_TOTAL) &lt;&gt; 0
                  order by 1 ) a,
                  (select distinct SNAP_ID,
                      to_char(END_INTERVAL_TIME,'DD-Mon-YYYY HH24:Mi') SNAP_TIME
                  from DBA_HIST_SNAPSHOT) b
              where a.snap_id=b.snap_id;
      Begin
          select sum(SPACE_ALLOCATED_DELTA) into v_TotalGrowth from DBA_HIST_SEG_STAT;
          select sum(bytes) into v_CurrentSize from dba_segments;
          v_BaselineSize := v_CurrentSize - v_TotalGrowth ;

          dbms_output.put_line('SNAP_TIME           Database Size(MB)');

          for row in usageHist loop
                  v_Space := (v_BaselineSize + row.ProgSum)/(1024*1024);
              dbms_output.put_line(row.SNAP_TIME || '           ' || to_char(v_Space) );
          end loop;
      end;
    /