Reorder Point & Safety Stock Calculator
Work out when to place your next order and how much safety stock protects you from demand and lead-time swings.
Quick answer
The reorder point is the stock level that triggers a new order: demand during lead time (daily demand x lead time days) plus safety stock, which is a z-score for the service level x demand variability.
Reorder Point & Safety Stock Calculator
Optional. Use 0 if demand is steady.
Optional. Use 0 if delivery time is fixed.
Reorder Point
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Safety Stock
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Demand During Lead Time
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Z-score
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Std. Dev. During Lead Time
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Safety stock by service level
Service Level Comparison
| Service level | Z-score | Safety stock (units) | Reorder point (units) | Extra stock vs. your level |
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Assumptions & method
- Demand during lead time = average daily demand × average lead time (L).
- Safety stock = z × √(L × σd² + d² × σL²), where σd is the standard deviation of daily demand, d the average daily demand and σL the standard deviation of lead time in days.
- Reorder point = demand during lead time + safety stock. Results are rounded up to whole units only in the headline note; the cards keep decimals.
- The z-score is the inverse of the standard normal distribution at your service level (a rational approximation with a relative error around 1e-9). It treats the service level as the chance of no stockout in a cycle.
- Assumes demand and lead time are independent and roughly normally distributed. Order size, review period, backorders and seasonality are not modelled.
Results are estimates based on the figures you enter. They are not guarantees and not financial advice.
What are the reorder point and safety stock?
The reorder point is the stock level that should trigger a new order so that goods arrive before you run out. It has two parts. The first is the stock you expect to sell while the order is on its way: average daily demand multiplied by lead time. The second is safety stock, a buffer for the times when demand is higher or deliveries are later than average. Its size depends on how variable demand and lead time are and on the service level you want. This calculator derives the z-score for your service level and shows how the buffer changes across service levels.