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Inventory Buffer & Reorder Models

Calculate a maximum-versus-average inventory buffer or a statistical safety stock using demand and lead-time variability.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Use the result with context

Results are estimates for planning and education. Confirm rates, taxes, fees, and legal requirements with the relevant institution or a qualified professional before making a financial decision.

Formula and method

Maximum demand of 120 units per day over five days, compared with average demand of 100 over three days, gives a 300-unit safety buffer and a 600-unit reorder point.

Buffer = maximum daily demand × maximum lead time − average daily demand × average lead time; statistical buffer = z × √(Lσd² + d²σL²)

Worked example

Enter these known values and leave the other values blank.

Max daily usage
120 units
Max lead time
5 days
Average daily usage
100 units
Average lead time
3 days
Safety stock
300 units
Reorder point
600 units

Assumptions and limitations

  • Maximum values cannot be below their averages. Equal maximum and average inputs are supported and can produce zero safety stock.
  • The statistical model uses average daily demand d, average lead time L, daily-demand standard deviation σd and lead-time standard deviation σL. It assumes the stated independent normal approximation; correlation and nonstationary demand require a different model.
  • The five service choices retain rounded z scores. Cycle service level is a probability of avoiding a stockout during a replenishment cycle, not the fraction of units immediately filled.
  • Demand quantities are per day. Lead-time selectors convert to days before calculation. The main result keeps the captured nearest-whole-unit rounding; a more precise calculated value appears under the field. Round upward to practical whole units or pack sizes when planning actual replenishment.

Common questions

What happens if lead-time variability is off?

The captured preset sets its standard deviation to zero, so only daily-demand variation contributes to the statistical buffer.

Are the two reorder methods interchangeable?

No. One uses maximum and average observations; the other uses a statistical approximation. They represent different assumptions and can produce different estimates.

References

Bookify allows maximum demand and lead time to equal their averages, including a zero safety-stock buffer.

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