Free safety stock calculator with reorder point

Enter your own numbers, click the button and read the result. These tools give estimates for planning only. They use simplified formulas and the figures you enter, so check important decisions against your own data and advisers.

Safety stock is the buffer you hold to cover variation in demand and in supplier lead time. The calculator combines both sources of variation into one number, then scales it by a z-score for the service level you choose: a higher service level means a larger z and more safety stock.

The reorder point is the stock level at which you place a new order. It equals the demand you expect during the lead time, plus the safety stock.

Worked example

This is an example only, using made-up round numbers to show the arithmetic. They are not benchmarks, and the results were produced by this page's calculator script.

Inputs: average daily demand 50 units, demand standard deviation 10 units per day, average lead time 9 days, lead time standard deviation 2 days, 95% service level (z = 1.65).

Results:

• Safety stock: 172.3 units (173 when rounded up)
• Demand during lead time: 450.0 units
• Reorder point: 622.3 units (623 when rounded up)

This safety stock calculator is most useful when:

• You are setting or reviewing reorder points for items that have variable demand or variable supplier lead times.
• Stockouts are frequent and you want a starting point for how much buffer to add.
• You are comparing the stock needed at different service levels before deciding what to hold.
• A supplier's lead time has changed and you need to see the effect on your reorder point.

If the numbers point to a bigger question, such as which suppliers, inventory policies or logistics routes to change, our supply chain consulting can help you work through it.

What is safety stock?

Safety stock is extra inventory held to protect against variation in demand and in supplier lead time, so that you can keep serving customers when things do not go to plan.

What is a reorder point and how does it relate to safety stock?

The reorder point is the inventory level at which you place a new order. It is the average demand during the lead time plus safety stock, so a larger safety stock means you reorder earlier.

Which service level should I choose?

It depends on the cost of a stockout compared with the cost of holding extra stock for that item. A higher service level needs more safety stock, so many teams choose different levels for different items.

Does this safety stock calculator work for every item?

It assumes demand and lead time are roughly normally distributed and independent. Items with very lumpy or intermittent demand, or with very little history, may need a different approach, so treat the result as a starting point to review.

What units should I use?

Use the same time unit for demand and lead time. This calculator uses units per day for demand and days for lead time, and the results are in units.

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