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The safety stock formula
Safety stock is the inventory you hold to survive the weeks when things go wrong. Too little and you run out during any ordinary disruption. Too much and your cash is sitting in a warehouse. Here is how to size it deliberately.
Last reviewed 16 August 2026
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What it is protecting you from
Safety stock covers two independent risks. Knowing which one you face changes the answer.
- Demand risk. You sell more than expected. A feature, a post that travels, a competitor going out of stock.
- Supply risk. Replenishment takes longer than planned. Production slips, freight is delayed, customs holds a shipment.
A product with steady demand and an unreliable overseas supplier needs buffer for the second. A viral product with a dependable local supplier needs it for the first. Products exposed to both need noticeably more than either alone.
The simple version
Good enough for most products and much better than nothing.
Safety stock = lead time demand × a buffer percentage
| Product profile | Buffer |
|---|---|
| Steady demand, reliable supplier | 20 to 25 percent |
| Some variability in either | 30 to 50 percent |
| Erratic demand or unreliable supply | 50 to 100 percent |
| Critical item you cannot be without | 100 percent or more |
Selling 8 a day with a 21 day lead time gives 168 units of lead time demand. At a 33 percent buffer that is 56 units of safety stock.
The statistical version
Worth doing for high value products where both over and under stocking are expensive.
Safety stock = Z × standard deviation of demand during lead time
Z is the service factor for the service level you want:
| Service level | Z | Meaning |
|---|---|---|
| 90 percent | 1.28 | Out of stock in roughly one cycle in ten |
| 95 percent | 1.65 | One in twenty |
| 98 percent | 2.05 | One in fifty |
| 99 percent | 2.33 | One in a hundred |
The cost curve is the important part. Moving from 95 to 99 percent is not a small step: it roughly doubles the buffer for a four percentage point gain. That is worth it on your best seller and wasteful on a long tail accessory.
The tradeoff nobody escapes
Safety stock is cash converted into risk reduction. It carries real costs: capital tied up, storage, insurance, obsolescence and the clearance discount at the end of a season.
The right level is where the marginal cost of one more unit of buffer equals the marginal cost of the stockouts it prevents. In practice nobody computes that exactly. What is achievable is to be deliberate: high buffer on the products where running out is expensive, low buffer on the ones where it is an inconvenience.
Applying one blanket percentage to everything is the same mistake as one blanket threshold: it overspends on the tail and underprotects the products that matter.
Then set the alert above it
Safety stock is not the level you get told about. It is the reserve you are trying not to eat into.
The alert belongs at the reorder point, which is lead time demand plus safety stock, so the order is placed while the buffer is still intact. See the reorder point formula.
Protect the buffer you paid for
Safety stock only works if something notices before you start consuming it. Stockwell holds your reorder point per SKU and emails your team the moment stock crosses it.
Get Stockwell on the Shopify App Store Free plan covers 50 SKUs with a daily summary. Paid plans add instant alerts and more recipients. No card needed to start.Common questions
How much safety stock should I hold?
For steady products with a reliable supplier, 20 to 25 percent of lead time demand. For erratic demand or unreliable supply, 50 to 100 percent. For critical items, more. The statistical version multiplies a service factor by the standard deviation of demand during lead time.
What service level should I target?
95 percent is a common default. Push to 98 or 99 only on products where running out is genuinely expensive, because the buffer roughly doubles between 95 and 99 percent for a small gain in coverage.
Is safety stock the same as a reorder point?
No. Safety stock is the reserve. The reorder point is lead time demand plus safety stock, so it is the higher number. You order at the reorder point so the safety stock stays untouched.