Query to report disk space allocation and used space

Question

We are using 6 databases in total for an application, and we can only share 4TB of space among all 6 auto-grow databases (via SAN storage).

I'd like to write a query (report) for a single database indicating the "Currently allocated space" and "Available free space" attributes under the Tasks > Shrink > Database option in SQL Server Management Studio.

Then I'd like to convert those numbers to TB and total each database to get a rough estimate of how much space we have left. Can these fields be accessed via a T-SQL query? If so, what would the query look like?