How to estimate the storage needed for a database?
The following would be an estimate that assumes full pages and doesn't take into consideration any database overhead (page header, etc.) or compression.  It would be simple math

5,000,000kb / 1024 = 4882.8125mb

4882.8125mb / 1024 = 4.76837158203125gb

But, I agree with other comments - you should create a test database and see how close the numbers match up.