What is safety stock?

Safety stock is extra inventory held beyond the expected demand during lead time. It acts as a buffer against demand spikes or delayed supplier deliveries, preventing stockouts and lost sales.

What does the service level (Z-score) mean?

The service level represents the probability of not stocking out during a replenishment cycle. A 95% service level means you will have enough stock to meet demand 95% of the time. Higher service levels require more safety stock.

What is the safety stock formula used here?

The basic formula is: Safety Stock = Z × σd × √LT, where Z is the service-level Z-score, σd is the standard deviation of demand, and LT is average lead time. When lead-time standard deviation is provided, the extended formula is: Z × √(LT × σd² + d² × σLT²), which accounts for both demand and lead-time variability.

What is the reorder point?

The reorder point (ROP) is the inventory level at which you place a new order. It equals average demand during lead time plus safety stock: ROP = (Average Daily Demand × Average Lead Time) + Safety Stock.

What time unit should I use?

Be consistent. If demand is in units per day, lead time must also be in days. If demand is per week, lead time must be in weeks. Mix units and the result will be wrong.

What if I do not know the standard deviation of demand?

A rough estimate is 20–30% of average demand. Alternatively, collect 8–12 weeks of historical sales data and calculate the standard deviation from that sample.

Safety Stock Calculator

Name: Safety Stock Calculator
Availability: InStock
Author: Nham Vu

Enter your demand and lead-time data to calculate the safety stock quantity you need to prevent stockouts at your desired service level.

Inventory Inputs

Average Demand (units per period)

Demand Std Deviation (units per period)

Average Lead Time (periods)

Lead Time Std Deviation (periods, optional)

Service Level (95% — Z = 1.645)

80% 85% 90% 95% 97.5% 99%

Fill in the inputs and click Calculate to see your safety stock.

Safety Stock

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units

Reorder Point

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units

Avg Lead Demand

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units

Formula Breakdown

Safety Stock by Service Level

Service Level	Z-Score	Safety Stock	Reorder Point

Summary

Enter your demand and lead-time data to calculate the safety stock quantity you need to prevent stockouts at your desired service level.

How it works

Enter the average daily (or weekly) demand for the product.
Enter the standard deviation of demand over the same time unit.
Enter the average supplier lead time in the same time unit.
Optionally enter the standard deviation of lead time for a more accurate result.
Choose a service level (fill rate) — higher service levels require more safety stock.
The calculator outputs safety stock quantity and the reorder point.

Use cases

Prevent stockouts during peak seasons without over-ordering.
Set reorder points that account for supplier variability.
Compare safety stock requirements across different service levels.
Justify buffer inventory to finance teams with a statistical basis.
Reduce carrying costs by right-sizing safety stock instead of guessing.
Plan inventory for new products based on forecast uncertainty.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu