The Excel FORECAST.ETS.SEASONALITY function returns the length of the repeating seasonal pattern that Excel detects in a time series — for example, 12 for a yearly cycle in monthly data.
Syntax
| Argument | Description | |
|---|---|---|
values | Required | The historical data range to analyse (e.g. monthly figures in B2:B40). |
timeline | Required | The matching, evenly spaced range of dates or numbers (e.g. A2:A40), the same size as values. |
data_completion | Optional | 1 (default) interpolates missing points; 0 treats gaps as zeros. |
aggregation | Optional | How duplicate timeline entries are combined (1 = AVERAGE, the default). |
How to use it
Before forecasting, it helps to know whether your data even has a season — and how long it is. FORECAST.ETS.SEASONALITY runs Excel's automatic detection and returns that length as a whole number:
A result of 12 on monthly data means each year repeats; 7 on daily data means a weekly rhythm; 4 on quarterly data means an annual cycle. A result of 0 (or 1) means Excel found no meaningful seasonality.
Tip: feed the number you get back into the seasonality argument of FORECAST.ETS to lock the cycle in place, rather than letting each call auto-detect independently. This keeps a whole dashboard of ETS formulas consistent.
The detection needs enough history to see at least a couple of full cycles — two years of monthly data, for instance. Too little history, or a flat/noisy series, returns 0.
Try it: interactive demo
Pick a FORECAST.ETS.SEASONALITY example to see the formula and its result.
Practice workbook
Frequently asked questions
What does the returned number mean?
0 means no seasonality was detected.Why does it return 0?
Should I use the result in FORECAST.ETS?
seasonality argument so every forecast uses the same fixed cycle length instead of re-detecting it. This is faster and keeps related formulas consistent.Which Excel versions support it?
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