FORECAST function
This article describes the formula syntax and usage of the FORECAST function in Microsoft Excel.
Description
Calculates, or predicts, a future value by using existing values. The predicted value is a yvalue for a given xvalue. The known values are existing xvalues and yvalues, and the new value is predicted by using linear regression. You can use this function to predict future sales, inventory requirements, or consumer trends.
Syntax
FORECAST(x, known_y's, known_x's)
The FORECAST function syntax has the following arguments:

X Required. The data point for which you want to predict a value.

Known_y's Required. The dependent array or range of data.

Known_x's Required. The independent array or range of data.
Remarks

If x is nonnumeric, FORECAST returns the #VALUE! error value.

If known_y's and known_x's are empty or contain a different number of data points, FORECAST returns the #N/A error value.

If the variance of known_x's equals zero, then FORECAST returns the #DIV/0! error value.

The equation for FORECAST is a+bx, where:
and:
and where x and y are the sample means AVERAGE(known_x's) and AVERAGE(known y's).
Example
Copy the example data in the following table, and paste it in cell A1 of a new Excel worksheet. For formulas to show results, select them, press F2, and then press Enter. If you need to, you can adjust the column widths to see all the data.
Known Y 
Known X 

6 
20 

7 
28 

9 
31 

15 
38 

21 
40 

Formula 
Description 
Result 
=FORECAST(30,A2:A6,B2:B6) 
Predicts a value for y given an x value of 30 
10.607253 