Safety Stock
Safety stock is a reserve buffer that cushions demand spikes and supplier delays so that shortages do not occur despite uncertainty. It is the insurance against stock-outs — and at the same time a cost driver, because it ties up capital in the warehouse. The right level balances service availability against carrying cost.
What is safety stock for?
- Demand fluctuations: unexpectedly high orders are covered
- Supplier delays: late receipts do not immediately cause a bottleneck
- Quality rejects: defective deliveries are bridged
Safety stock feeds directly into the reorder point.
Calculating safety stock
A simple approximation multiplies average daily usage by a risk reserve in days:
Safety stock = average daily usage × risk days
Statistically more precise is the calculation via the desired service level (the safety factor, or Z-score) and the standard deviation of demand and lead time. A higher service level (e.g. 98% instead of 95%) requires disproportionately more stock.
Service level and the trade-off
- High safety stock: high availability but a lot of tied-up capital and carrying cost
- Low safety stock: low cost but a risk of shortages and stock-outs
An ABC analysis helps concentrate scarce reserve on the items that matter most.