Computed Column – PERSISTED and Storage

If we are creating one computed column in the database it won't take more size of that computed column until we give the PERSISTED keyword

It does not increase the data size of the original table as well as it does not turn the column into persisted. When we create an index on the column of the table, there is an additional space occupied by that index.

More Details:
http://blog.sqlauthority.com/2010/08/21/sql-server-computed-column-persisted-and-storage-part-2/