Q:

Excel shows incorrect polynomial trendline function

I'm trying to create a polynomial function that fits my data in column H.  Column I are the x values, and column J is the function that the charts polynomial trendline shows. So why is my result (see orange line) not equal to the trendline function? What's going on here?

A:

This is due to the way Excel calculates the coefficients for an nth order Polynomial Trendline and the precision used in coming up with the coefficients. The discrepancy is due to the rounding precision of the coefficients and only comes about when one tries to calculate future values manually using the equation.
Due to the rounding precision, the coefficients cannot properly be used to manually calculate y values. However, the Polynomial Trendline shown on the chart is precise and you can make a future prediction just by looking at the chart.
Chart trendline formula is inaccurate in Excel

