Safety stock is the buffer you hold on top of expected demand to absorb the unexpected — a sales spike, or a supplier who ships late. Too little and you stock out; too much and you tie up cash. Here's how to size it.
The simple safety stock formula
For most small and medium stores, a practical buffer is a few days of average sales:
Safety stock = average daily sales × buffer days
Example: a product sells 5/day and you want a 3-day buffer → safety stock = 5 × 3 = 15 units. Simple, and good enough for the majority of SKUs.
The advanced formula (service level)
If demand or lead time is volatile, size the buffer to a target service level using variability:
Safety stock = Z × σ_demand × √(lead time)
- Z — the service-level factor (e.g. 1.65 for ~95%, 2.33 for ~99%).
- σ_demand — the standard deviation of daily demand.
- lead time — average replenishment time in days.
Higher service level → more buffer → fewer stockouts, but more cash tied up. Reserve high service levels for your best-sellers and high-margin items.
How safety stock fits the bigger picture
Safety stock feeds directly into your reorder point:
Reorder point = (sales velocity × lead time) + safety stock
So getting safety stock right is half of knowing exactly when to reorder. It builds on your sales velocity.
Best practices
- Bigger buffer for volatile or high-margin products; a stockout on your best seller costs the most.
- Smaller buffer for steady, low-margin items to free up cash.
- Revisit seasonally — demand variability changes through the year.
Calculating this per product by hand doesn't scale. Foreshelf lets you set a buffer per shop, supplier or product and applies it automatically across your whole catalogue.