Demand forecasting
Seasonality
By the Supply Chain Manager team · Updated October 16, 2026
Seasonality is a repeating, predictable pattern in demand tied to the calendar, such as higher sales every winter, every month-end or every Monday.
Seasonal cycles can be yearly (weather, holidays), monthly (budget cycles) or weekly (trade customers ordering on Mondays). A seasonal pattern repeats at a known interval; a one-off spike does not, and should not be treated as seasonal.
Seasonal indices express each period as a multiple of the average: an index of 1.4 for December means December demand runs 40% above a typical month.
How to calculate seasonality
Seasonal Index = Average demand in that period ÷ Average demand across all periods
Seasonality example
Illustrative numbers: a heating product averages 1,000 units a month across the year, 1,800 in January and 400 in July. January's index is 1.8 and July's is 0.4. Planning July purchases from the annual average would leave 600 surplus units on the shelf.
Why seasonality matters for distributors
Seasonal SKUs need stock built before the peak, which means ordering at least one lead time earlier. Missing the pattern leads to stockouts in the peak and overstock afterwards.
Common seasonality mistakes
- Calling a pattern seasonal after seeing it only once.
- Using a plain average for a seasonal SKU.
Examples on this page use made-up round numbers to show the calculation. They are not benchmarks or results from real businesses.