Excel trend line intercept

Question

I have an Excel graph with a linear trend line to keep track of users who are updated with a newer version of software:

I have 660 users, and the trend line predicts where the number updated reaches 660 to indicate updates complete. Is there a way for it to either give me an actual value for that intercept, or, more conveniently, draw a vertical intercept line where the trend line is projected to hit that number?

Answer 1

If you're looking for a predicted DATE when the Number Updated reaches 660, you need to write some formulas.

I did this in a spreadsheet I was using to track Backup growth at a previous job. The solution does not involve the graph, it uses the FORECAST() function of Excel.

The syntax is:

=FORECAST(X, Yrange, XRange)

In your case X equals 660, Yrange is the Date, and Xrange is the Number of Successful Updates.

So if your dates are in column A and the TOTAL number of successful updates is in column B, and you've got less than a thousand values entered already, you'll put the following formula in a spare cell somewhere:

=FORECAST(660, A1:A1000, B1:B1000)

In my experience I was unable to tell it to just use all column values (A:A) without it failing somehow, so I had to set some arbitrary limit (A1:A1000) for it to work.

Format the cell you put this formula in as Date and you'll get a prediction as to when your number of installs will equal the number of users.

Excel Tips was a great help in finding this information for me back when I was building the spreadsheet myself: http://excel.tips.net/T002573_Using_the_FORECAST_Function.html