Warehouse Calculators

Safety Stock Calculator

Calculate safety stock and reorder point from average daily demand, demand variability, lead time and target service level.

Demand

units
units

Replenishment

days

Service level

Safety Stock

185.2 units

Reorder point 1,865.2 units at a 95% service level

Safety stock

185.2units

Expected lead-time demand

1,680.0units

Reorder point

1,865.2units

Service level

95%

Z value

1.65

Lead time

14days

Simplified safety-stock models rely on assumptions about demand distribution and lead time. Actual inventory policy may require more advanced modelling, and this result is not a guarantee of inventory optimization.

The Safety Stock Formula

This calculator uses the standard statistical model for variable demand with a fixed lead time:

Safety stock = Z × σ(daily demand) × √(lead time in days)

Z is the service factor taken from the standard normal distribution: 1.28 for 90%, 1.65 for 95%, 1.96 for 97.5% and 2.33 for 99%. Because demand variability accumulates over the lead time as the square root rather than linearly, doubling the lead time increases safety stock by roughly 41%, not 100%.

Reorder point

Reorder point = average daily demand × lead time + safety stock. The first term is the expected demand during the replenishment lead time; the safety stock covers demand above that expectation.

Worked example

With average daily demand of 120 units, a daily standard deviation of 30 units, a 14-day lead time and a 95% service level: safety stock = 1.65 × 30 × √14 ≈ 185 units. Lead-time demand is 120 × 14 = 1,680 units, so the reorder point is approximately 1,865 units.

Assumptions and limitations

  • Daily demand is assumed to be approximately normally distributed and independent day to day.
  • Lead time is assumed fixed and known; lead time variability is not modelled.
  • The service level is a cycle service level, not a fill rate.
  • Seasonality, promotions, supplier minimums and shelf life are not considered.

Simplified safety-stock models rely on these assumptions, and actual inventory policy may require more advanced modelling. Treat the output as a starting point for review rather than a guaranteed optimum.

Related Pages

Frequently Asked Questions

What is the safety stock formula used here?
Safety stock = Z × standard deviation of daily demand × the square root of the lead time in days. This is the standard statistical model for variable demand with a fixed, known lead time.
What Z value corresponds to my service level?
The calculator uses Z = 1.28 for 90%, 1.65 for 95%, 1.96 for 97.5% and 2.33 for 99%. These are the standard normal values for a one-sided cycle service level.
How is the reorder point calculated?
Reorder point = average daily demand × lead time in days + safety stock. It is the inventory level at which a replenishment order should be raised.
Does this model account for lead time variability?
No. This version assumes a fixed lead time and variable demand. When lead time itself varies materially, a combined model that includes lead time standard deviation is more appropriate.
Is a higher service level always better?
No. Safety stock rises steeply as the service level approaches 100%, so the inventory cost of the last few percentage points is often larger than the shortage cost it prevents.

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