TREND function
This article describes the formula syntax and usage of the TRENDfunction in Microsoft Excel.
Description
Returns values along a linear trend. Fits a straight line (using the method of least squares) to the arrays known_y's and known_x's. Returns the yvalues along that line for the array of new_x's that you specify.
Syntax
TREND(known_y's, [known_x's], [new_x's], [const])
The TREND function syntax has the following arguments:

Known_y's Required. The set of yvalues you already know in the relationship y = mx + b.

If the array known_y's is in a single column, then each column of known_x's is interpreted as a separate variable.

If the array known_y's is in a single row, then each row of known_x's is interpreted as a separate variable.


Known_x's Required. An optional set of xvalues that you may already know in the relationship y = mx + b.

The array known_x's can include one or more sets of variables. If only one variable is used, known_y's and known_x's can be ranges of any shape, as long as they have equal dimensions. If more than one variable is used, known_y's must be a vector (that is, a range with a height of one row or a width of one column).

If known_x's is omitted, it is assumed to be the array {1,2,3,...} that is the same size as known_y's.


New_x's Required. New xvalues for which you want TREND to return corresponding yvalues.

New_x's must include a column (or row) for each independent variable, just as known_x's does. So, if known_y's is in a single column, known_x's and new_x's must have the same number of columns. If known_y's is in a single row, known_x's and new_x's must have the same number of rows.

If you omit new_x's, it is assumed to be the same as known_x's.

If you omit both known_x's and new_x's, they are assumed to be the array {1,2,3,...} that is the same size as known_y's.


Const Optional. A logical value specifying whether to force the constant b to equal 0.

If const is TRUE or omitted, b is calculated normally.

If const is FALSE, b is set equal to 0 (zero), and the mvalues are adjusted so that y = mx.

Remarks

For information about how Microsoft Excel fits a line to data, see LINEST.

You can use TREND for polynomial curve fitting by regressing against the same variable raised to different powers. For example, suppose column A contains yvalues and column B contains xvalues. You can enter x^2 in column C, x^3 in column D, and so on, and then regress columns B through D against column A.

Formulas that return arrays must be entered as array formulas.
Note In Excel Web App you cannot create array formulas.

When entering an array constant for an argument such as known_x's, use commas to separate values in the same row and semicolons to separate rows.
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.
The first formula shows corresponding values to the known values. The second formula predicts the next months values, if the linear trend continues.


Note The formula in the example must be entered as an array formula in the Excel desktop application. After copying the example to a blank worksheet, select the range C2:C13 or B15:B19 starting with the formula cell. Press F2, and then press CTRL+SHIFT+ENTER. If the formula is not entered as an array formula, the single results are 133953.3333 and 146171.5152.