Purchase Order Lead Time & Safety Buffer Planner
Calculate safety stock and reorder points using lead time and demand variability for optimal inventory buffers
Worked Examples
Example 1: Manufacturing Buffer Planning
Problem: Component lead time 30 days (±5 days std dev), demand 100 units/day (±20 std dev), target 95% service level. Calculate safety stock and reorder point.
Solution: Inputs:\n- Avg lead time (L): 30 days\n- Lead time std dev (σL): 5 days\n- Avg demand (D): 100 units/day\n- Demand std dev (σD): 20 units/day\n- Service level: 95% (Z = 1.65)\n\nSafety Stock Calculation:\nFormula: SS = Z × √(L × σD² + D² × σL²)\n= 1.65 × √(30 × 20² + 100² × 5²)\n= 1.65 × √(30 × 400 + 10,000 × 25)\n= 1.65 × √(12,000 + 250,000)\n= 1.65 × √262,000\n= 1.65 × 511.86\n= 844.6 units\n\nReorder Point:\nROP = (D × L) + SS\n= (100 × 30) + 845\n= 3,000 + 845\n= 3,845 units\n\nInterpretation:\n- When inventory drops to 3,845, place order\n- Normal consumption during 30 days: 3,000 units\n- Safety buffer: 845 units (8.45 days demand)\n- Service level: 95% (1 stockout per 20 cycles)\n\nCost Analysis (assuming $10/unit value, 25% holding cost):\n- Safety stock value: 845 × $10 = $8,450\n
Result: Safety stock: 845 units (8.5 days) | ROP: 3,845 units | 95% service level | $2,113 annual holding cost
Frequently Asked Questions
What is purchase order lead time?
Lead time is duration from placing PO (purchase order) to receiving goods. Includes: Supplier processing (1-3 days), manufacturing/picking (varies), shipping (3-30 days), customs (international, 3-14 days). Example: Order on Day 1, receive Day 30 = 30-day lead time. Variability matters: if lead time is '30 days ±10', you must buffer for worst case (40 days) to avoid stockouts. Reducing lead time or variability reduces inventory requirements.
What is safety stock?
Safety stock is buffer inventory to prevent stockouts from demand spikes or supply delays. Formula: Z-score × √(Lead time × Demand variance + Demand² × Lead time variance). Z-score depends on service level (95% service = 1.65). Higher safety stock = fewer stockouts but higher holding costs. Trade-off: Cost of carrying inventory vs. cost of lost sales. Optimize: balance holding cost with stockout cost.
What causes lead time variability?
Lead time variation sources: (1) Supplier capacity (busy periods = delays), (2) Shipping disruptions (weather, port congestion, carrier issues), (3) Customs (random inspections, paperwork errors), (4) Quality issues (rejected batches, rework), (5) Order prioritization (supplier delays your order for larger customer). Reduce variability: (1) Supplier relationship (priority customer status), (2) Dual-source (backup supplier), (3) Premium shipping (faster, more reliable), (4) Near-shore vs. offshore (shorter, less variable).
How do I reduce safety stock without stockouts?
Options: (1) Reduce lead time (faster supplier, air vs. ocean freight), (2) Reduce lead time variability (reliable supplier, premium shipping), (3) Reduce demand variability (better forecasting, smooth production), (4) Lower service level (accept occasional stockouts if cost justified), (5) Just-in-time (JIT) delivery (supplier ships small batches frequently). Toyota JIT: Near-zero safety stock but requires reliable suppliers and stable demand. Most companies: Balance—maintain some buffer.
What is EOQ and how does it relate to safety stock?
EOQ (Economic Order Quantity) optimizes order size (how much to order). Safety stock optimizes inventory level (when to order). EOQ balances: ordering cost (fixed per order) vs. holding cost (variable per unit). Formula: EOQ = √(2 × Demand × Order cost / Holding cost). Safety stock is separate: buffer against uncertainty. Together: Order EOQ quantity when inventory hits ROP (which includes safety stock). EOQ minimizes cost of ordering + holding. Safety stock minimizes stockout risk.
Should I buffer lead time or safety stock?
Same effect, different implementation. Lead time buffer: Order earlier (when inventory is higher). Safety stock buffer: Order at normal reorder point but keep more inventory. Mathematically equivalent: 5-day lead time buffer = 5 days of demand as safety stock. Practical difference: Lead time buffer is proactive (order early), safety stock is reactive (maintain buffer). Most use safety stock (easier to manage one variable—inventory level—than multiple: timing + level).