I don't delete my full backup history so that I can use previous backup size to predict future growth.

-- Average DB size by month for ALL Databases

-- Backups DO NOT include free space in database

selectsubstring(a.database_name,1,35) as 'Database',

datepart(year,a.backup_start_date) as 'year',

datepart(month,a.backup_start_date) as 'month' ,

avg(cast((a.backup_size /1073741824) as decimal (9,2)))as 'Avg Gig'

FROM msdb.dbo.backupset a

join msdb.dbo.backupset b on a.server_name = b.server_name

and a.database_name = b.database_name

WHERE a.type = 'D' and b.type = 'D'

--and a.database_name = 'xyz'

and a.backup_size > 1073741824 -- > 1 Gig

GROUP BY a.database_name,datepart(year,a.backup_start_date),datepart(month,a.backup_start_date)

order by a.database_name,datepart(year,a.backup_start_date) desc,datepart(month,a.backup_start_date) desc