How do I figure out how much database is expected to grow?

Is this an Access mdb or accdb ?

You might want to consider whether there are going to be any changes  in working practices. New employees hired, big sakes promotion, new direction of work,  or a new feed of data now someone has decided that text stores well and perhaps they could store pictures.
Make an estimate by looking at the monthly growth and then double it to add a safety margin. Storage is becoming cheaper all the time so aim for the highest you can afford if you decide to increase it.

" big Sales promotion

"big Sales promotion": what if they sell sake?

Are the numbers you provided the size of the database each month or the increase in size from the previous month?

Saw the typoo and tried to change it but the edit function was broken.

At a base level, that's primarily a math q, and many math folks could give you very detailed answers on trending, stat analysis, etc..

In a db context, that's too small to spend that much time worrying about.  I think this is related to your SQL Express q, and I think I might've taken care of that now too :-).

you can try  Bitsqueezer suggestion : "Excel" chart to review and analyze based on your processes, and business specifics

more:

Building a simple forecast solution on SQL Server by Terry Choo
http://terrychoo.com/forecast/

Forecasting growth
http://searchsqlserver.techtarget.com/feature/Forecasting-growth

Hi Experts,

So far I tested bisqueezers suggestion and seems to work, would someone know whats the calculation behind it?
(the links provided are way to complicated for me to follow..)

Thanks,
Ben

Is there any database maintenance being done? What purging happens, on what schedule? Are reorgs done? (Does any maintenance reflect in the given figures?) Was the initial size "3330" MB, or is that only the oldest size known?

Hi Experts,
fyi- what I'm concerned re the DB see https://www.experts-exchange.com/questions/29013683/Database-maintenance.html?anchor=a42079051¬ificationFollowed=186938705&anchorAnswerId=42079051#a42079051
Re the logic, I see its not something simple I can figure out just by doing some plus & minuses, so I guess will stick with what excel is giving w/o looking into the details..

on what schedule?

We have a daily backup scheduled.

or is that only the oldest size known?

Yes, this is the case.

Thanks,
Ben

Thank you very much my experts!