FORECAST function
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)
X is the data point for which you want to predict a value.
Known_y's is the dependent array or range of data.
Known_x's is 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
The example may be easier to understand if you copy it to a blank worksheet.
How to copy an example

Create a blank workbook or worksheet.

Select the example in the Help topic.
Note Do not select the row or column headers.
Selecting an example from Help

Press CTRL+C.

In the worksheet, select cell A1, and press CTRL+V.

To switch between viewing the results and viewing the formulas that return the results, press CTRL+` (grave accent), or on the Formulas tab, in the Formula Auditing group, click the Show Formulas button.

