Inventory glossary

What is Forecast error?

Forecast error is the gap between the sales quantity you predicted for a SKU and what actually sold in that period.

Forecast error tells you how much to trust your reorder numbers. If your forecasts are consistently off by a wide margin, your purchase orders will be wrong too, leading to either stockouts or cash stuck in excess stock.

Track it per SKU, not just store-wide. A new product and a long-running bestseller behave differently, and averaging their errors together hides which items actually need a closer look or a safety stock adjustment.

Formula

Forecast error = Actual sales - Forecasted sales

Example

A candle store forecasts 100 units of a vanilla candle for March. Actual sales come in at 80 units. Forecast error is 80 - 100 = -20 units, meaning the forecast overestimated demand by 20 units.

Common mistakes

  • Looking only at total store sales instead of per-SKU error, which hides problem items.
  • Ignoring the sign of the error, so overstocking and understocking patterns get mixed together.
  • Not adjusting future forecasts after spotting a repeated error in the same direction.

Forecast error and SteadyShelf

SteadyShelf builds its per-variant forecasts from your actual sales history, so lower forecast error over time means more accurate order-by dates and reorder quantities.

SteadyShelf is launching on the Shopify App Store. Join early access to get the install link first.

Related terms

Demand forecasting · Forecast accuracy · Forecast bias · Mean absolute deviation · MAPE

Full inventory glossary

FAQ

Is some forecast error normal?

Yes. No forecast is exact. The goal is to keep the error small and watch for patterns, not to eliminate it completely.

What is the difference between forecast error and forecast bias?

Forecast error is the gap in one period. Forecast bias is what happens when that error leans the same direction repeatedly, like always forecasting too high.

Let SteadyShelf do this math for every SKU

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