2nd Polynomial Trendline prediction

I've got a record of Heating Oil usage and I'm trying to extrapolate the date when it predicts I'll run out. I'm using a 2nd order Polynomial because it best matches the consumption line.
Oil Usage 1.xlsx

I'm trying to use the LINEST function to do this.
I tried

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but it won't let me include "#N/A" cells.

I tried

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but it also won't let me include "#N/A" cells.

I clearly don't understand how it works.

Is this the best/correct way to do it please?

I've already posted to https://www.mrexcel.com/board/thread...point.1226329/ but the reply advised the use of an Add-in which I tried to install but it didn't work. I'm running a fragile Windows 7 / Office 2010 and am wary about adding/installing anything that would risk it's operation. I'd prefer to use the standard formulas and equations if possible.

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I have your solution. But... you MUST first tell us where else you have cross-posted this.

OK. Try:

=LINEST(C2:INDEX(C:C,MATCH(10^10,E:E)),E2:INDEX(E:E,MATCH(10^10,E:E))^{1,2})

You seem to be using an older version of Excel than me. So, please refer to the attached file. If the formulae are enclosed within a pair of { }, these are array formulae.

These are a little different from ordinary formulae in that they MUST be confirmed in the FIRST CELL ONLY by pressing CTRL+SHIFT+ENTER to activate the array, not just ENTER.

You will know the array is active when you see the curly brackets { } appear around the outside of your formula. If you do not use CTRL+SHIFT+ENTER you will (almost always) get an error message or an incorrect answer. Press F2 on that cell and try again.

Don't type the curly brackets yourself - it won't work...

You must pay careful attention to exactly what you are doing.

Your original LINEST() functions (and the chart trendline regression) were regressing y=volume against x=date serial number (y=ax^2+bx+c)
Glenn's formula is regression x=date serial number against y=volume. x=ay^2+by+c. In this equation, c is indeed the x intercept for polynomial, but it's not the same x intercept as you would get regressing volume against date.

It should be as easy as switching the column C and column E references in the INDEX() functions (not the MATCH() functions) in Glenn's formula

=LINEST(E2:INDEX(E:E,MATCH(10^10,E:E)),C2:INDEX(C:C,MATCH(10^10,E:E))^{1,2})

Then you should get the same polynomial coefficients as the chart trendline, and you should then be able to use the quadratic formula to find the x intercepts of the polynomial.

Maybe you can use the equation as Excel can show on the chart.
You should set the accuracy high, so use scientific with 15 decimal digits.
You can manually copy the equation from the chart and paste it into a cell
with string formulas you can explode the equation into its parameters
the parameters can be used to plot youre trendline.
in the attachment i used VBA macro to get the equation out of the chart.
The rest you can see in the sheet......
Hope this helps