Demand Forecasting › Forecast Error Cost
Forecast Error Cost: MAPE Impact on Inventory and Sales
Forecast error has two distinct cost components: over-forecast creates excess inventory that incurs carrying cost and markdown risk; under-forecast creates stockouts that lose gross margin and customers. Most retailers focus on one side and underweight the other.
Two-sided cost formula
Total Error Cost = Over-Forecast Cost + Under-Forecast Cost
Over-Forecast Cost = Excess Inventory $ × (Carrying % + Markdown Cost %)
Under-Forecast Cost = Lost Unit Sales × (Gross Margin per Unit + Churn-Adjusted Future Sales)
MAPE (Mean Absolute Percentage Error) is the most-used accuracy metric. It does not distinguish over from under, so two operators with identical MAPE can have very different cost profiles depending on which side dominates their errors.
MAPE benchmarks by context
| Context | Typical MAPE | Note |
|---|---|---|
| Grocery (centre store, weekly) | 8-12% | Stable demand; high SKU count balances. |
| Apparel (weekly, full season) | 20-30% | Style-level uncertainty; new SKUs each season. |
| Electronics (promotional) | 25-40% | Promo events drive demand swings of 5-10x. |
| Toys (Q4) | 30-50% | Hit-driven; long lead times amplify error. |
| Home improvement (seasonal) | 15-25% | Weather-correlated; storm-prep over- and under-buys. |
| Best-in-class consumer goods ML | 5-10% | Companies running modern demand-sensing platforms (RELEX, o9, Blue Yonder). |
Worked example: apparel buyer, 25% MAPE
A $200M apparel buyer runs 25% MAPE. Inventory base $50M. 45% gross margin. Half of error is over, half under.
Over-forecast side
- Excess inventory = 12.5% of $50M = $6.25M
- Carrying + markdown = 35% / year
- Annual cost = $2.19M
Under-forecast side
- Lost sales = 12.5% × $200M = $25M
- Lost margin = 45% × $25M = $11.25M
- Churn-adjusted (15% recapture loss) = +$3.75M
- Annual cost = $15.0M
Total annual forecast error cost = $17.2M, of which approximately 87% comes from the under-forecast side. Improving MAPE from 25% to 15% would recover roughly $6.9M of margin annually. That economics typically justifies an enterprise demand-planning platform investment of $1-3M.