This article describes the formula syntax and usage of the FORECAST function (function: A prewritten formula that takes a value or values, performs an operation, and returns a value or values. Use functions to simplify and shorten formulas on a worksheet, especially those that perform lengthy or complex calculations.) 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 (argument: A value that provides information to an action, an event, a method, a property, a function, or a procedure.):
 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. If you are copying the example in Excel Online, copy and paste one cell at a time.
Important: Do not select the row or column headers.
Selecting an example from Help
 Press CTRL+C.
 Create a blank workbook or worksheet.
 In the worksheet, select cell A1, and press CTRL+V. If you are working in Excel Online, repeat copying and pasting for each cell in the example.
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.

A 
B 
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 (10.60725) 
