Oracle How to calculate export dump size

Sometimes you have to export all the database or many schemas but you don't have all the necessary space needed, or you don't know exactly how much space the dmp file it will take. 

Oracle with exports (or expdp) stores at the dmp file the table data. Indexes are just "Create index statements" which will be created after loading the table data. So the key factor of how big will be your dmp file, depends on how much table data you have. 

The following query calculates how much table data each schema takes on your database

SELECT owner, ROUND(SUM(size_mb)) MBytes FROM 
(
SELECT owner, segment_name, segment_type, partition_name,

ROUND(bytes/(1024*1024),2) SIZE_MB,

tablespace_name 
FROM DBA_SEGMENTS 
WHERE SEGMENT_TYPE IN ('TABLE', 'TABLE PARTITION',

 'TABLE SUBPARTITION') 
--AND TABLESPACE_NAME LIKE 'COSTE%' 
--AND SEGMENT_NAME LIKE 'OE_ORDER_LINES_ALL%' 
--AND partition_name LIKE 'USAGE_FCT_NEW%'
--AND OWNER = 'TARGET_DW' 
--AND ROUND(bytes/(1024*1024),2) > 1000)
) 
GROUP BY owner 
ORDER BY MBytes DESC;

OWNER                              MBYTES
------------------------------ ----------
TARGET_DW                         3774208
TARGET_POC                         673192
STAGE_DW                           469263
PRESTAGE_DW                        389326
SHADOW_DW                          257233
PRESENT_PERIF                      148063
SNAP                               141565
PKIOUSIS                           117535
DM_SPSS                             44760
MONITOR_DW                          35336
CUSTOMER_VIEW                       29807

for example if you export the SHADOW_DW schema, it will create a dmp file approximately 257233MB->250GBytes. 

Tip: 
Bear in mind that the actual dump file will have smaller size than 250GB!!!, because in most cases the TABLE DATA are fragmented!. Export-Datapump, removes fragmentation!. 

Indexes, packages, procedures, views, etc, are not taking too much space in the dmp file, are just DDL statements.

SELECT 'expdp SYSTEM/oracle DUMPFILE=' || owner || '_%U.dmp

 DIRECTORY=exp_dir PARALLEL=10 LOGFILE='

 || owner ||'.log schemas= ' || owner SQL
FROM (
SELECT owner, ROUND(SUM(size_mb)) MBytes FROM 
(
SELECT owner, segment_name, segment_type, partition_name,

 ROUND(bytes/(1024*1024),2) SIZE_MB,

 tablespace_name 
FROM DBA_SEGMENTS 
WHERE SEGMENT_TYPE IN ('TABLE', 'TABLE PARTITION',

 'TABLE SUBPARTITION') 
--AND TABLESPACE_NAME LIKE 'COSTE%' 
--AND SEGMENT_NAME LIKE 'OE_ORDER_LINES_ALL%' 
--AND partition_name LIKE 'USAGE_FCT_NEW%'
AND OWNER <> 'SYSTEM' 
--AND ROUND(bytes/(1024*1024),2) > 1000)
) 
GROUP BY owner 
ORDER BY MBytes DESC);