Seasonality Index Calculator

A seasonal index turns 12 months of raw numbers into a simple scale where 100 means average. Anything above 100 is a stronger than average month, anything below is weaker, which makes it easy to spot your real peak and trough months instead of eyeballing a chart.

How the index is calculated

This is the classic average method used in demand forecasting: add up all 12 months, divide by 12 to get the yearly average, then divide each month's value by that average and multiply by 100. A month that comes in exactly at the yearly average scores 100. A month running 20% above average scores 120, and one running 20% below scores 80.

What data to feed it

Traffic, revenue, order count, leads, or search volume for a specific keyword all work, since the formula doesn't care what the underlying metric is. What matters more is using the same metric consistently and, if you have more than one year of history, averaging each month across years before entering it here so a single unusual month doesn't distort the index.

Using the swing between peak and trough

The gap between your highest and lowest index points tells you how seasonal your business actually is. A small swing, say under 20 points, suggests fairly steady demand year round, while a large swing suggests you should plan budget, staffing, content and inventory around a concentrated period rather than spreading resources evenly across the year.