Oracle Database Size

SELECT    ROUND(SUM(USED.BYTES) / 1024 / 1024 / 1024 ) || ' GB' "Database Size"
,    ROUND(SUM(USED.BYTES) / 1024 / 1024 / 1024 ) -
    ROUND(FREE.P / 1024 / 1024 / 1024) || ' GB' "Used space"
,    ROUND(FREE.P / 1024 / 1024 / 1024) || ' GB' "Free space"
FROM    (SELECT    BYTES
    FROM    V$DATAFILE
    UNION    ALL
    SELECT    BYTES
    FROM     V$TEMPFILE
    UNION     ALL
    SELECT     BYTES
    FROM     V$LOG) USED
,    (SELECT SUM(BYTES) AS P
    FROM DBA_FREE_SPACE) FREE
GROUP BY FREE.P;