Lead time is an important measure in supply chain management. It affects inventory planning, production schedules, customer delivery commitments and working capital. When suppliers take longer than expected to deliver goods, businesses may need to hold additional inventory or risk running out of stock.
1. What Is Lead Time?
Lead time is the time taken to complete a process from a defined starting point to a defined ending point.
In supply chain management, supplier lead time commonly refers to the time between placing a purchase order and receiving the goods. Depending on the business, the measurement may end when goods arrive, when they are received into the warehouse, or when they become available for use.
Lead Time = End Date and Time − Start Date and Time
The start and end points should be defined consistently. Otherwise, comparisons between suppliers or reporting periods may be misleading.
2. Types of Lead Time
Supplier lead time
The time between placing an order with a supplier and receiving the goods, based on the agreed measurement points.
Production lead time
The time required to manufacture a product or complete a production process.
Order processing lead time
The time required to process an order, including activities such as order entry, verification and preparation.
Transportation lead time
The time required to move goods between locations.
Customer order lead time
The time between a customer placing an order and receiving the product or service, depending on the definition used by the business.
3. Practical Example: Calculating Supplier Lead Time
A company places a purchase order on 1 October. The supplier delivers the goods to the receiving warehouse on 11 October.
If the company measures lead time in elapsed calendar days, excluding the starting date, the calculation is:
11 October − 1 October = 10 calendar days
The supplier lead time is therefore 10 calendar days, assuming the goods arrive on 11 October and the order was placed on 1 October at the same time of day.
If the company measures business days instead of calendar days, the result may differ because weekends and public holidays may be excluded.
4. Understanding the Components of Lead Time
A purchase order may involve several stages before the goods are available for use.
| Activity | Example Duration |
|---|---|
| Supplier order processing | 2 days |
| Production or preparation | 4 days |
| Transportation | 3 days |
| Receiving and inspection | 1 day |
| Total sequential lead time | 10 days |
In this example, the activities are assumed to occur sequentially without overlap. In actual operations, some activities may happen simultaneously, so the total elapsed time should be measured from the actual start to the actual end rather than simply adding every activity duration.
5. How Lead Time Affects Inventory
Businesses need to consider lead time when deciding when to reorder stock. Longer lead times generally mean that replenishment orders must be placed earlier.
A basic reorder point calculation is:
Reorder Point = Demand During Lead Time + Safety Stock
For constant daily demand: Reorder Point = Daily Demand × Lead Time in Days + Safety Stock
Suppose a business sells 50 units per day. Its supplier lead time is 10 days, and the business maintains safety stock of 100 units.
Demand during lead time = 50 × 10 = 500 units
Reorder Point = 500 + 100
Reorder Point = 600 units
Under these assumptions, the business should trigger replenishment when inventory reaches approximately 600 units, provided the demand, lead time and safety stock assumptions remain appropriate.
6. What Happens When Lead Time Increases?
Suppose daily demand remains at 50 units and safety stock remains at 100 units, but supplier lead time increases from 10 days to 15 days.
Demand during lead time = 50 × 15 = 750 units
Reorder Point = 750 + 100
New Reorder Point = 850 units
The longer lead time increases the reorder point by 250 units.
This does not mean every business should simply increase inventory. Management should investigate whether the lead time can be reduced, whether demand can be forecast more accurately, and whether a different replenishment arrangement would be more effective.
7. Lead Time Variability
Average lead time alone does not show how reliable a supplier is. A supplier may deliver in 10 days on average but sometimes take 18 days.
Lead time variability can create additional inventory requirements because businesses need to protect themselves against unexpected delays.
For example, two suppliers may have the following performance:
| Measure | Supplier A | Supplier B |
|---|---|---|
| Average lead time | 10 days | 10 days |
| Observed delivery range | 9–11 days | 7–18 days |
Both suppliers have the same average lead time in this illustration, but Supplier A is more consistent. Supplier B’s greater variability may require additional planning or safety stock, depending on demand patterns and the consequences of a stockout.
8. How to Reduce Supply Chain Lead Time
- Measure each stage of the purchasing and delivery process.
- Improve purchase order accuracy and approval turnaround.
- Work with suppliers to improve production planning and delivery reliability.
- Review transport routes and shipping arrangements.
- Reduce unnecessary receiving and inspection delays without compromising quality.
- Improve demand forecasting and share forecasts with suppliers.
- Monitor actual lead time against the supplier’s committed lead time.
9. Useful Lead Time KPIs
Businesses can monitor several indicators to understand supply chain performance:
- Average lead time: Average elapsed time between the defined start and end points.
- Lead time variability: How much actual lead time varies across orders.
- On-time delivery rate: The percentage of deliveries received by the agreed date.
- Supplier delay: Actual delivery date compared with the promised delivery date.
- Stockout frequency: How often inventory is unavailable when required.
These measures should be reviewed together. Reducing average lead time is useful, but delivery reliability and product availability also matter.
10. Related Resources
Conclusion
Lead time affects inventory availability, purchasing decisions and customer service. By measuring lead time consistently, tracking supplier reliability and incorporating replenishment time into inventory planning, businesses can reduce delays and improve supply chain performance.
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