Seasonal Adjustment Calculator Formula

Understand the math behind the seasonal adjustment calculator. Each variable explained with a worked example.

Use the Seasonal Adjustment Calculator

Formulas Used

Seasonally Adjusted Value

adjusted = observed / seasonal_index

Seasonal Effect

seasonal_effect = observed - (observed / seasonal_index)

Seasonal Effect (%)

pct_seasonal = (seasonal_index - 1) * 100

Variables

VariableDescriptionDefault
observedObserved Value150
seasonal_indexSeasonal Index1.25

How It Works

How to Seasonally Adjust Data

Formula

Adjusted Value = Observed Value / Seasonal Index

Seasonal adjustment removes predictable seasonal patterns from data, revealing the underlying trend. A seasonal index above 1 means the season inflates the value; below 1 means it deflates it. Dividing by the index normalizes the value as if there were no seasonal effect.

Worked Example

December sales are $150 with a seasonal index of 1.25 (holiday boost).

observed = 150seasonal_index = 1.25
  1. 01Adjusted = 150 / 1.25 = 120
  2. 02Seasonal effect = 150 - 120 = 30
  3. 03The seasonal boost accounts for $30 (25%) of the observed value
  4. 04The underlying (de-seasonalized) value is $120

Ready to run the numbers?

Open Seasonal Adjustment Calculator