if I fit a polynomial of degree three I get a nice curve with equation 4e-06x^3-0.0003x^2+0.019x+0.2813

however when I put this in formula I get completely different curve. Any idea why this is happening?

The error is due to the rounded coefficients. You need to format the trendline "label" (equation) so it displays more precision.

I like to always display 15 significant digits using the Scientific format. To do that: right-click on the trendline "label", clilck on Format Trendline Label, Number, Scientific, enter 14 into Decimal Places, and click on OK.

Then you can copy-and-paste each coefficient from the "label" into Excel cells. Be mindful of the sign of the coefficient. If the equation is of the form 4e-06x^3 - 0.0003x^2..., the second coefficient is -0.0003, not 0.0003.

(I assume you will use SUMPRODUCT or SERIESSUM to apply the coefficients to your data in Excel. Alternatively, you can write a formula that is similiar to the trendline equation, of course.)

Alternatively, you might consider using LINEST to derive the coefficients directly in Excel instead of relying on the chart trendline. Generally, for an order-3 polynomial, you would select 4 horizontal cells, then

array-enter a formula of the following form (

press ctrl+shift+Enter instead of just Enter):

=LINEST(Y1:Y71, X1:X71^{1,2,3})

If you want to use that approach, but you need more specifics,

__we__ need more specifics. The best thing to do is: upload an example Excel file to a file-sharing website, and post the public/share URL in a response here. The following is a list of some free file-sharing websites.

Dropbox: http://dropbox.com

Box.Net: http://www.box.net/files

MediaFire: http://www.mediafire.com

FileFactory: http://www.filefactory.com

FileSavr: http://www.filesavr.com

RapidShare: http://www.rapidshare.com

Windows OneDrive: http://onedrive.live.com