The inventory level at which a new order should be placed to receive stock before running out.

Simple: SS = Z × σ × √LT. For spare parts: typically 1-3 units based on criticality.

Use maximum monthly demand instead of average. Increase safety stock for critical items.

For lead times >6 months, consider keeping minimum 1 spare on shelf for critical items regardless of demand.

Balance holding cost (~20-25% of value/year) against stockout cost (production loss). For critical spares, stockout cost usually far exceeds holding cost.