Seasonal index
A seasonal index is a number that shows how a calendar month's sales compare with sales in an average month of the year.
How it works
The index turns seasonality into one number per calendar month, measured against the year’s average month. An index of 1.0 is an average month; 1.2 means 20% above it. The twelve indices add up to 12.
One year mixes the season with the store’s growth and one-off events: in a growing store, later months look strong because the store is bigger. Classical decomposition first removes this trend by detrending: it divides each month by a centred 12-month moving average. Then it averages each calendar month’s ratios across years and scales them to total 12. Calculating a seasonal index applies the method to your own sales.
Formula
Sales in the month ÷ average monthly sales for the year
Where you see it
- GetProfit portal: Assortment and seasonality compares revenue by month over 24 completed months and says whether the second year repeated the first.
Example
Example store, not client data.
The tableware shop averages 180,000 of revenue a month. November brings 288,000: 288,000 ÷ 180,000 = 1.6. July brings 135,000: 135,000 ÷ 180,000 = 0.75.
Not to be confused with
- Peak-to-trough ratio — one number for the whole year: the best months against the worst. The index gives each month its own.
- Seasonality adjustment — a Google Ads setting that warns Smart Bidding of a short jump or drop in conversion rate. It is not calculated from sales.
Right and wrong readings
- Wrong: “June comes out at 0.4, so June is our off-season.” Right: check that the month is complete. In a first run on GetProfit data, 51 of 100 stores showed a June 2026 dip; for 72 stores the data ended on 10 June. The cause was a cut-off export, not demand.
- Wrong: “One year of sales gives us our seasonal index.” Right: one year shows each month once, so a deep dip cannot be told from a one-off event. Comparing the same month year over year shows whether the pattern repeats.
Benchmarks
In GetProfit data on 96 stores (July 2025 – June 2026, detrended monthly ad revenue), the three best months brought a median 41.7% of a store’s yearly ad revenue; an even year would give 25%. The figure covers one cycle of ad revenue only. Your niche may differ.
Sources
- 3.4 Classical decomposition — Forecasting: Principles and Practice (3rd ed), Hyndman and Athanasopoulos: detrending and seasonal indices. Checked 2 October 2026.
- GetProfit data: 96 online stores, July 2025 – June 2026, detrended monthly ad revenue.