How to calculate it
Sales lost = normal units per day × days unavailable − extra units similar lines sold
- Find the product's normal units sold per day from comparable days when it was available, such as the same weekdays in the previous few weeks.
- Multiply by the days it was unavailable. That is the unserved demand at the normal rate.
- Subtract the extra units similar lines sold in the same store over those days. What remains is an estimate of sales that left the store; multiply by gross profit per unit to value it.
Illustrative example
Three empty days on a coffee line
A coffee line sells about 12 bags a day at one store. It is unavailable from Thursday to Saturday, so the unserved demand at its normal rate is 12 × 3 = 36 bags. Over the same three days the store's other coffees sold 10 more bags than usual, so about 26 sales left the store. At $6 gross profit a bag, that is about $156.
The bigger cost is often in the data. The three zero days look like three days nobody wanted the product, and a forecast that learns from them orders less next time. Flag them as unavailable before the next forecast or range review uses that week.
Check before acting
- Take the normal rate from comparable days. A promotion week inflates it, and a week with an earlier gap deflates it.
- Count only extra sales of genuine substitutes in the same store. Unrelated lines that sold more prove nothing about where these shoppers went.
Questions about this calculation
- How do you calculate lost sales from a stockout?
- Multiply the normal units sold per day by the days unavailable, then subtract the extra units similar lines sold over those days. The remainder estimates the sales that left the store.
- Why subtract sales of similar lines?
- Some shoppers buy a substitute instead. Those sales stayed in the store, so counting them as lost would overstate the cost of the stockout.
- Does this include shoppers who never come back?
- No. A shopper who switches stores after a gap can cost future visits too. This estimate covers only the days the product was unavailable.
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