FORECAST function
This article describes the formula syntax and usage of the FORECASTfunction 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
The example may be easier to understand if you copy it to a blank worksheet.
How do I copy an example?

Select the example in this article.
Important Do not select the row or column headers.
Selecting an example from Help

Press CTRL+C.

In Excel, create a blank workbook or worksheet.

In the worksheet, select cell A1, and press CTRL+V.
Important For the example to work properly, you must paste it into cell A1 of the worksheet.

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.
After you copy the example to a blank worksheet, you can adapt it to suit your needs.

