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# Excel FORECAST Function

Related Function:

TREND functionForecast Equations

The Excel Forecast Function calculates the new y-value from the simple equation for a straight line :

where,

and

and the values of

x and

y are the sample means (the averages) of the known_x's and the known_y's.

## Basic Description

The Excel Forecast function predicts a future point on a linear trend line fitted to a supplied set of x- and y- values.

The syntax of the function is:

FORECAST( x, known_y's, known_x's )

Where the arguments are as follows :

x | - | A numeric x-value for which you want to forecast a new y-value |

known_y's | - | An array of known y-values |

known_x's | - | An array of known x-values. Note that the length of the known_x's array should be the same length as known_y's, and the variance of the known_x's must not be zero. |

## Forecast Function Example

The spreadsheet below shows an example of the Forecast function used to predict an additional point along the straight line of best fit through the known_x's and known_y's in cells F2:F7 and G2:G7. These known points are shown in the graph on the same spreadsheet.

The above example gives the result *32.666667*.

Further examples of the Excel Forecast function can be found on the Microsoft Office website

## Forecast Function Errors

If your Forecast function gives you an error message, use the table below to look up the likely cause:

Common Errors

#VALUE! | - | Produced if the supplied future x value is non-numeric |

#NA | - | Occurs if there are problems with the supplied known_x's or known_y's arrays - i.e. if either of these arrays is empty, or if the arrays are of different lengths. |

#DIV/0! | - | Produced if the variance of the supplied known_x's evaluates to zero. |