Analyze demand variability, exposure periods, and stockout probability. Model reorder decisions with practical inventory metrics. Plan smarter replenishment cycles for steadier customer order fulfillment.
Use safety stock for a calculated reorder point, or enter a manual reorder point to test an existing policy.
This chart shows how cycle service level changes as reorder point shifts around expected exposure demand.
| Scenario | Avg Demand/Day | Demand SD | Lead Time | Review Period | Safety Stock | Reorder Point | Cycle Service Level |
|---|---|---|---|---|---|---|---|
| Fast Movers | 120.00 | 25.00 | 5.00 days | 2.00 days | 90.00 | 930.00 | 74.44% |
| Seasonal Item | 80.00 | 30.00 | 7.00 days | 3.00 days | 140.00 | 940.00 | 77.42% |
| Premium SKU | 20.00 | 6.00 | 14.00 days | 0.00 days | 35.00 | 315.00 | 70.76% |
| Steady Essentials | 200.00 | 18.00 | 3.00 days | 1.00 days | 60.00 | 860.00 | 71.38% |
The calculator measures the probability of avoiding a stockout during the protection period. In ecommerce inventory control, that probability is the cycle service level.
1) Exposure period
Exposure Period = Lead Time + Review Period
2) Mean demand during exposure
Mean Exposure Demand = Average Daily Demand × Exposure Period
3) Demand variability during exposure
Exposure Standard Deviation = √[(Exposure Period × Daily Demand SD²) + (Average Daily Demand² × Lead Time SD²)]
4) Reorder point
Reorder Point = Mean Exposure Demand + Safety Stock
If a manual reorder point is entered, that value is used instead.
5) Service factor
z = (Reorder Point − Mean Exposure Demand) ÷ Exposure Standard Deviation
6) Cycle service level
Cycle Service Level = Φ(z)
Φ(z) is the cumulative normal probability.
7) Expected shortage per cycle
Expected Shortage = Exposure Standard Deviation × [φ(z) − z × (1 − Φ(z))]
8) Recommended reorder point for a target
Required Safety Stock = z(target) × Exposure Standard Deviation
Recommended Reorder Point = Mean Exposure Demand + Required Safety Stock
Cycle service level is the probability that inventory will not stock out during one replenishment cycle. It focuses on whether a cycle ends without a stockout event.
Cycle service level measures the chance of no stockout in a cycle. Fill rate measures the percentage of unit demand fulfilled immediately. The two metrics answer different inventory questions.
Periodic review systems only check inventory at intervals. That adds extra exposure time, so demand uncertainty must cover both review time and supplier lead time.
Even stable demand can cause stockouts when lead times fluctuate. Longer or inconsistent replenishment windows increase the amount of demand you must protect with safety stock.
Use safety stock when you want the calculator to derive the reorder point. Use a manual reorder point when testing an existing policy or comparing current settings with target service goals.
Not always. Higher service levels reduce stockout risk, but they usually raise inventory carrying cost. The best setting balances customer experience, working capital, and demand volatility.
Many sellers target roughly 90% to 98%, depending on margin, item criticality, supplier reliability, and customer expectations. Fast movers or strategic products often justify stronger protection.
Yes. Set review period to zero for continuous review. Enter a positive review period when replenishment decisions happen at fixed intervals rather than continuously.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.