Seasonal Sales Forecast Calculator
Project monthly sales targets using seasonal indexes and year-over-year growth. Pre-filled with typical retail seasonality.
What is Seasonal Indexing?
A seasonal index represents how a given month's sales compare to the monthly average. An index of 100 means that month is exactly average. An index of 150 means sales are 50% above the monthly average (like December in retail). An index of 75 means sales are 25% below average.
How the forecast is calculated:
- Project the new annual total: Last Year Sales × (1 + Growth %)
- Calculate each month's share: Index ÷ Sum of All Indexes
- Monthly Forecast = Annual Total × Monthly Share
The pre-filled indexes reflect typical US retail seasonality — slow in winter (Jan/Feb), a spring bump, steady summer, back-to-school lift in August/September, and a peak in November/December driven by holiday shopping.
You can customize the indexes based on your own historical data. If January has historically been 20% above your monthly average, set it to 120. If it's 30% below, set it to 70.