If you only have SMS tablespaces (system managed tablespaces) then you can take the total pages times the page size. If you had any DMS tablespaces, there is a difference between allocated space (total pages) and used space (used pages), but for your database that is not a factor.
Temporary tablespaces grow and shrink as they are used by DB2 for queries, sorts, utlilities, etc.
In addtion to the tablespaces, there is disk space used for the logs. If you have circular logging, you can calculate that based on the database config parms for number of primary and seconday log files, and the size of each file. For archive logging (need for roll forward recovery), you may also have archived logs stored on disk.