What safety stock is (and isn't)
Safety stock is the buffer quantity of a product you keep on hand beyond what you expect to sell before your next delivery arrives. It exists specifically to cover two kinds of uncertainty: demand variability (you sell more than expected in a given period) and supply variability (your supplier delivers later than promised).
It is not the same as your reorder point, though the two are related. The reorder point is the stock level that triggers a new order; safety stock is one of the components used to calculate that reorder point, alongside expected demand during lead time.
A basic safety stock formula
A commonly used formula is: Safety Stock = (Maximum Daily Usage × Maximum Lead Time in Days) − (Average Daily Usage × Average Lead Time in Days). This method accounts for the worst realistic case on both demand and lead time, rather than just the average.
Smaller businesses often use a simpler rule instead — for example, holding safety stock equal to a fixed number of days of average demand, or a flat percentage on top of expected usage. The more precise formula is worth the extra effort for high-value or frequently-stocked-out items; a simpler rule is often good enough for the rest of your catalog.
- Precise formula: accounts for both maximum demand and maximum lead time
- Simple rule: a fixed number of days (or percentage) of average demand
- Higher demand variability or longer/unreliable lead times both push safety stock higher
Setting safety stock levels in practice
In StockFlow, you set a minimum stock level per product this functions as your reorder trigger, and the buffer you build into it above your genuinely expected baseline demand is your safety stock. StockFlow will send a low-stock email alert once quantity on hand reaches that threshold, so setting the number thoughtfully (not just defaulting to a round number) is what actually makes the alert useful.
A practical starting point: look at your worst month of demand for that product over the past year, and your worst-case (not average) supplier delivery time, then set your minimum stock level to cover that gap. Revisit it periodically stock that never seems to trigger an alert may be over-buffered, tying up cash unnecessarily.
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