Calculate seasonal indices and deseasonalise a series.
An index above one marks a peak season and below one a trough. Dividing observed values by their index removes the seasonal pattern so the underlying trend becomes visible. Comparing raw month-on-month figures in a seasonal business is meaningless; deseasonalising is what makes the comparison valid.
Seasonality Index
Seasonal index = season value ÷ overall average; indices across a cycle average to 1
Seasonal index = season value ÷ overall average; indices across a cycle average to 1 An index above one marks a peak season and below one a trough. Dividing observed values by their index removes the seasonal pattern so the underlying trend becomes visible.
Comparing raw month-on-month figures in a seasonal business is meaningless; deseasonalising is what makes the comparison valid.
This calculator takes 4 inputs: Value for this season, Average across all seasons, Forecast average for next cycle, Seasons per cycle. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.