How do I calculate tables size in Oracle

This query tells you how much space is allocated for each table including any indexes” and “LOBs” rather than just the table itself.
Note: This requires access to the DBA_* views.

COLUMN TABLE_NAME FORMAT A32
COLUMN OBJECT_NAME FORMAT A32
COLUMN OWNER FORMAT A10

SELECT
   owner,
   table_name,
   TRUNC(sum(bytes)/1024/1024) MB,
   ROUND( ratio_to_report( sum(bytes) ) over () * 100) Percent
FROM
(SELECT segment_name table_name, owner, bytes
 FROM dba_segments
 WHERE segment_type IN ('TABLE', 'TABLE PARTITION', 'TABLE SUBPARTITION')
 UNION ALL
 SELECT i.table_name, i.owner, s.bytes
 FROM dba_indexes i, dba_segments s
 WHERE s.segment_name = i.index_name
 AND   s.owner = i.owner
 AND   s.segment_type IN ('INDEX', 'INDEX PARTITION', 'INDEX SUBPARTITION')
 UNION ALL
 SELECT l.table_name, l.owner, s.bytes
 FROM dba_lobs l, dba_segments s
 WHERE s.segment_name = l.segment_name
 AND   s.owner = l.owner
 AND   s.segment_type IN ('LOBSEGMENT', 'LOB PARTITION')
 UNION ALL
 SELECT l.table_name, l.owner, s.bytes
 FROM dba_lobs l, dba_segments s
 WHERE s.segment_name = l.index_name
 AND   s.owner = l.owner
 AND   s.segment_type = 'LOBINDEX')
WHERE owner in UPPER('<Schema_Name>')
GROUP BY table_name, owner
HAVING SUM(bytes)/1024/1024 > 10  /* Ignore really small tables */
ORDER BY SUM(bytes) desc;