typically, the compressed, persisted data that Splunk extracts from your data inputs amounts to approximately 10% of the raw data that comes into Splunk. the indexes that are created to access this data can be anywhere from 10% to 110% of the size of the data that comes in. this value is affected strongly by how many unique terms occur in your data.
(from http://www.splunk.com/base/Documentation/latest/Installation/HowHowmuchspaceyouwillneed )
typically, the compressed, persisted data that Splunk extracts from your data inputs amounts to approximately 10% of the raw data that comes into Splunk. the indexes that are created to access this data can be anywhere from 10% to 110% of the size of the data that comes in. this value is affected strongly by how many unique terms occur in your data.
(from http://www.splunk.com/base/Documentation/latest/Installation/HowHowmuchspaceyouwillneed )