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The FORECAST.ETS.SEASONALITY function returns the length of a seasonal pattern based on existing values and a timeline.

Note: This FORECAST.ETS function is only available in Excel 2016 and later versions, and not available in Excel for the Web, iOS, or Android.


FORECAST.ETS.SEASONALITY(values, timeline, [data_completion], [aggregation])


  • Values (required): The existing or historical known values for which you want to predict the next point (y values);
  • Timeline (required): A range of date/time or numeric values corresponding to the “Values” (x values);
  • Data_completion (optional): A numeric value specifies how to handle the missing points in the timeline. It can be:
0: Missing points will be treated as zeros;
1 or omitted: Missing points will be calculated as the average of the neighboring points.
  • Aggregation (optional): A numeric value specifies which function will be used to aggregate several values with the same time stamp. The values and the corresponding functions are listed below.
 Numeric value  Function
 1 or omitted  AVERAGE
 4  MAX
 6  MIN
 7  SUM


1. If no repetitive seasonal pattern is detected, it will return zero;
2. The seasonal pattern returned by the FORECAST.ETS.SEASONALITY function is the same as that is calculated automatically by the FORECAST.ETS function (the argument “seasonality” is defined as 1);
3. The #N/A error occurs if “values” and “timeline” arrays are not the same size;
4. The #NUM! error occurs when any of the following conditions is met:
-- All “timeline” values are the same;
-- A constant step cannot be identified in the provided timeline;
-- “data_completion” is any number other than 0 or 1;
-- “aggregation” is out of the valid range (1-7).
5. The #VALUE! error occurs when [data_completion] or [aggregation] is nonnumeric.

Return value

It returns a positive integer.


As shown in the table below, you have applied the FORECAST.ETS function (the formula is displayed below) to forecast the sales for January to July 2022 based on the monthly sales for 2021.


In the formula, the argument “seasonality” is specified as 1, which tells Excel to automatically detect the seasonality and define a proper length of the seasonal pattern. To know the length of this seasonal pattern automatically specified by Excel, you can do as follows to get it done.

1. Select a cell (E6 in this case), copy or enter the formula below and press the Enter key to get the result.


Related Functions

Excel FORECAST function
The FORECAST function predicts a future value based on existing values by using the linear regression.

Excel FORECAST.ETS function
The FORECAST.ETS function uses the Exponential Smoothing (ETS) algorithm to predict a future value based on a series of existing values.

The FORECAST.ETS.CONFINT function calculates the confidence interval for the forecast value at the specified target date.

Excel FORECAST.ETS.STAT function
The FORECAST.ETS.STAT function returns a specified statistical value as a result of time series forecasting.

Excel FORECAST.LINEAR function
The FORECAST.LINEAR function predicts a future value based on existing values by using the linear regression.

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