Free safety stock calculator

This free safety stock calculator turns demand swings and supplier lead time into a buffer you can order against.

Free safety stock calculator: z-score × demand standard deviation × √lead time. Pick a service level. Runs in your browser, nothing uploaded.

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Your numbers

Use the same unit and the same kind of day for demand and lead time. Numbers only, up to 6 decimal places, no commas. The standard deviation is how far daily demand usually moves away from its average.

Safety stock: 24.62 units

Rounded up: 25 units.

Add this to your lead-time demand to get the reorder point. It is a buffer, not an order quantity.

How the safety stock calculator works

You enter three things: how much your daily demand moves around its average, how many days your supplier takes to deliver, and the service level you want. The calculator then applies this formula: safety stock = z × σ × √lead time.

The z-score comes from the service level. 90% gives 1.28, 95% gives 1.64, 97.5% gives 1.96 and 99% gives 2.33. A higher service level means a bigger buffer. Everything runs in your browser. Nothing you type is sent or saved.

What each input means

  • Standard deviation of daily demand (σ): how far daily sales usually fall above or below the average. In a spreadsheet, use the STDEV function on several months of daily sales.

  • Lead time: the days from placing the purchase order until the stock can be sold. Use the same kind of day as your demand.

  • Service level: the chance that you do not run out during one reorder cycle. It is not the same as fill rate, which counts units shipped.

Worked example

σ = 4 units a day, lead time = 14 days, service level = 95%. 1.6449 × 4 × √14 = 1.6449 × 4 × 3.742 = 24.6, so hold 25 units.

At 99%, z is 2.3263 and the buffer is 34.8, so 35 units. Going from 95% to 99% adds 10 units, about 41% more safety stock, for 4 more points of protection. That is why the service level is a cash decision, not only a service decision.

Now add the buffer to your lead-time demand. If you sell 10 units a day on average, the reorder point is 10 × 14 + 25 = 165 units. The reorder point calculator does that last step for you.

When this formula is wrong

  • Demand comes in lumps, such as a few big orders a month. The formula assumes a bell curve, and lumpy demand does not follow one.

  • Your supplier is often late. The formula assumes a fixed lead time. If lead time varies, you need a version that also uses the lead time spread.

  • Your sales history includes stockouts. Days with zero sales because you were out of stock make σ look smaller than real demand.

  • The item is seasonal or growing. A trend inflates σ, so use forecast error instead of raw sales.

  • The item is new, or you have only a few weeks of history. σ from a short history is not reliable.

  • Demand and lead time use different units, for example σ per week and lead time in days.

Where to go next

FAQ

Frequently asked questions

It depends on what a stockout costs you. Many teams use a higher level, such as 97.5% or 99%, for items that earn the most or that customers cannot do without, and 90% to 95% for slow, low-margin items.

No. Safety stock is the extra buffer. The reorder point is the stock level at which you order, and it equals demand during the lead time plus safety stock.

Yes, if both inputs use the same period. If σ is per week, enter the lead time in weeks, for example 2 weeks instead of 14 days.

No. The calculation runs in your browser. Nothing is uploaded, stored or tracked.

Start with your workflow

Bring one inventory, purchasing or order-management problem. We will map the required data, demonstrate the relevant workflow and identify what needs configuration or further validation.

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