Diskspace analysis, database growth (yearly)

I am using the following  query to forecast the yearly database growth


select cast(MAX(Space_Used_in_MB)as float) as max_size , min(Space_Used_in_MB) AS MIN_SIZE ,

cast((MAX(Space_Used_in_MB) - MIN(Space_Used_in_MB)) as decimal(18,2))/ MIN(Space_Used_in_MB) * 100 as [yearly percent growth] ,

Database_Name from SQLMonitor..SQLAdmin_Monitor_db_file_info

where FILE_ID = 1

and Time_collected >='01/13/2014'

group by Database_Name

order by database_name


--SQLMonitor is a user database
--SQLAdmin_Monitor_db_file_info  is a user table where a job ( code here) logs the database file sixe in every four hours

-- if the time frame you want to measure is two years change the time_collected to '01/13/2013'  and change the yearly percent growth calculation as follows

( cast((MAX(Space_Used_in_MB) - MIN(Space_Used_in_MB)) as decimal(18,2))/ MIN(Space_Used_in_MB) * 100 )/2

Growth of database ( http://www.sqlservercentral.com/articles/Administration/analyzingdiskcapacityforsqlserver/2467/)

Database growth can be of three types:

1. linear growth (grows at a constant amount over time)
2. compound growth (grows at a constant rate for a specific period of time)
3. geometric growth (grows periodic by some increment, which is in turn growing at a constant rate)

Below are the formulae for different type of growths:
Linear: Current disk space + (growth * No of periods)
Compound: Current disk space * (1 + Growth %) ^ No of periods
Geometric: Current growth + [initial increment * {1-Incrementgrowth rate^ (No of period+1)}]/ (1 – growth rate)
To illustrate the fact, let us take one example. Say for, you have a database and it is of 100 GB. If it grows at 10% per year, then after 2 years, it will be 100 + (10 * 2) = 120 GB (linear growth).
If it is so that the database grows at 5% rate/month for 2 years, then it will be
100 * (1 + 0.05) ^ 24 = 322.5 GB (compound growth).
Take another case. Say for you have already a database of 100GB. Now the growth rate is so that it starts at 10 GB/month and the increment itself increments at 5%/month, then in 24 months, it will be 100 + {(10 * (1- (1.05 ^ (24 + 1))) / (1-1.05)} = 100 + 477 = 577 GB (geometric growth).