About the Lead Time Calculator
In inventory management, lead time is the time between placing an order with a supplier and having the goods ready to sell or use. It includes the supplier processing the order, making or picking the goods, and shipping them to you. Knowing lead time is essential for deciding when to reorder: order too late and you run out of stock; order too early and cash is tied up in inventory sitting on shelves.
This lead time calculator adds up the stages of lead time, then uses your daily demand and its variability to work out the safety stock needed for a chosen service level and the reorder point, the stock level at which you should place the next order.
How to Use the Lead Time Calculator
Enter the days for order processing, production and transit.
Enter the average daily demand and its standard deviation.
Choose the service level, the chance of not running out during the lead time.
The Formulas
lead time = processing + production + transit
demand in LT = daily demand × lead time
safety stock = z × σ(daily demand) × √lead time
reorder point = demand in LT + safety stockz values: 90% 1.28, 95% 1.645, 98% 2.05, 99% 2.33.
Step-by-Step Example
2 days processing, 10 production, 5 transit; 40 units a day with a deviation of 8; 95% service.
lead time 2 + 10 + 5 = 17 days
demand in LT 40 × 17 = 680 units
safety stock 1.645 × 8 × √17 ≈ 55 units
reorder point 680 + 55 = 735 unitsWhy Safety Stock?
Demand is rarely exactly average. Some days you sell more, some less. If you reorder only when stock equals the average demand during the lead time, you run out about half the time. Safety stock is a buffer that covers higher-than-average demand while waiting for the order. The service level sets how much protection you want: 95% means you expect to run out in about one replenishment cycle in twenty.
The Square Root Rule
Daily variations partly cancel out over several days, so the uncertainty in total demand grows with the square root of the lead time, not in proportion to it. That is why safety stock uses √lead time. A lead time four times longer needs only twice the safety stock, all else being equal.
Measuring Demand Variability
Calculate the standard deviation of daily sales from recent history, for example the last 60 or 90 days, using a spreadsheet. Exclude unusual events such as one-off bulk orders, or plan for them separately. For seasonal products, use data from the same season.
Variable Lead Times
The formula assumes the supplier always delivers on time. If deliveries vary, add a term for lead-time variation: safety stock = z × √(LT × σd² + d² × σLT²), where σLT is the standard deviation of lead time and d is average daily demand. A simpler approach is to plan with a longer lead time that suppliers meet most of the time.
Reducing Lead Time
Shorter lead times reduce both the stock used while waiting and the safety stock needed. Options include local suppliers, faster shipping, ordering more often in smaller amounts, electronic ordering, sharing forecasts so suppliers can plan production, and keeping key components in stock with the supplier.
Lead Time in Other Contexts
In manufacturing, lead time can mean the time from receiving a customer order to delivering it, and in services the time from request to completion. The same idea applies: break the time into stages, measure each, and focus improvement on the longest or most variable.
Economic Order Quantity
The reorder point says when to order; the order quantity says how much. A classic method, the economic order quantity, balances the cost of placing orders against the cost of holding stock: EOQ = √(2 × yearly demand × cost per order ÷ holding cost per unit per year). Combining a reorder point with a sensible order quantity gives a simple, reliable replenishment system for many small businesses.
Review Periods
If you check stock only periodically, for example every Monday, rather than continuously, you must cover demand for the review period as well as the lead time. Add the review period to the lead time in the calculation, so a weekly review with a 17-day lead time uses 24 days.
Multiple Suppliers
When the same item comes from more than one supplier, calculate a reorder point for each with its own lead time, or use the longest lead time to stay safe. Splitting orders between suppliers can also reduce the risk of a single late delivery causing a stockout, at the cost of more administration.
Keeping Figures Current
Review lead times and demand every few months. Suppliers change their schedules and sales patterns shift, so reorder points calculated once and never revisited soon lead to either stockouts or excess inventory.
Lost Sales
Remember that stockouts cost more than the missed sale: customers may go elsewhere for good.
Understanding Your Result
The headline gives the total lead time.
The breakdown line shows each stage.
The reorder point line gives the stock level at which to order.
The safety stock line shows the buffer and service level.
When Should You Use This Calculator?
Use it to set reorder points in an inventory system.
Use it to see how supplier delays affect stock needs.
Use it to compare suppliers with different lead times.
Common Mistakes
Ignoring safety stock. Average demand runs out half the time.
Forgetting processing time. Lead time starts when you order.
Using stale demand data. Recalculate as sales change.
Assuming lead times never vary. Plan for late deliveries.