Calculate Database Size in SQL server Syntax

One of the best way to calculate size of all database using SQL server management studio query editor.

SELECT      sys.databases.name,
            CONVERT(VARCHAR,SUM(size)*8/1024)+' MB' AS [Total disk space]
FROM        sys.databases 
JOIN        sys.master_files
ON          sys.databases.database_id=sys.master_files.database_id
GROUP BY    sys.databases.name
ORDER BY    sys.databases.name

--or 

exec sp_spaceused 

You may look for Calculate all tables Size in database in SQL server Syntax.