Seasonal index: Definition, Formula & Why It Matters

A seasonal index is a multiplier that captures how much a period’s demand deviates from the average. An index of 1.0 is average, 1.3 is 30% above, 0.7 is 30% below. It adjusts a baseline forecast for predictable seasonal swings.

Seasonal index formula

Seasonal index = average demand for the period ÷ overall average demand

Example

A December index of 1.8 means you plan for nearly double your normal demand that month.

Why it matters for Reverb sellers

Music gear has real seasonality: holidays, tax-refund season, back-to-school. A seasonal index keeps you from understocking your peak and overstocking your trough.

How Verbstack helps

Verbstack reveals your shop’s seasonal patterns so you can stock ahead of your busy periods.

Track this on every order, automatically.

  • Real fees, margins, and profit on every Reverb sale
  • COGS and inventory tracked for you, no spreadsheet
  • Full history and a live monthly P&L
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See this number on every order.

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