The amount of disk space needed for the indexes= sum of all the maxTotalDataSizeMB of all the indexes + 2GB safety.
If you are using the Volume option, read this http://docs.splunk.com/Documentation/Splunk/latest/Indexer/Configureindexstoragesize
If you are splitting the indexes homePath and ColPath in different partitions, see this http://wiki.splunk.com/Deploy:BucketRotationAndRetention
and http://docs.splunk.com/Documentation/Splunk/latest/Indexer/Usemultiplepartitionsforindexdata
the formula for the space used by one index on each location is :
homePath total size =(maxWarmDBCount + maxHotBuckets ) * maxDataSize
coldPath total size = maxTotalDataSizeMB - "size of the hot+warm+cold buckets"
If you also have $SPLUNK_HOME on the same partition, do not forget to account for the dispatch folder (depends of the roles, usually 100MB per user + 500MB per power user + 10000MB per admin) (see authorize.conf)
