Reorder Point Calculator
Calculate the reorder point for inventory replenishment using average daily demand, lead time, and safety stock to prevent stockouts.
Lead Time Demand Calculator
Calculate inventory demand during supplier lead time. Essential input for reorder point calculations. Accounts for daily demand and lead time variability.
units
days
Daily demand variability
units
Supply variability
days
For adequacy check
units
Lead Time Demand⧉
1,000.00 units
100.00 units/day × 10.00 days
LTD Std Deviation⧉
±210 units
Coefficient of variation: 21.00%
Buffer Above LTD⧉
+500.00 units
5.0 extra days of supply
95% ROP⧉
1,346.00 units
LTD 1,000.00 + SS 346.00
Stock vs Lead Time Demand
0LTD: 1,000.00Stock: 1,500.00
Reorder Point by Service Level
| Service Level | Z | Safety Stock | ROP | Stock Status |
|---|---|---|---|---|
| 85% | 1.04 | 218.00 | 1,218.00 | +282.00 |
| 90% | 1.28 | 269.00 | 1,269.00 | +231.00 |
| 95% | 1.65 | 346.00 | 1,346.00 | +154.00 |
| 97.5% | 1.96 | 411.00 | 1,411.00 | +89.00 |
| 99% | 2.33 | 489.00 | 1,489.00 | +11.00 |
| 99.9% | 3.09 | 648.00 | 1,648.00 | -148.00 |
Demand Scenario Analysis
| Daily Demand | LTD | σ LTD |
|---|---|---|
| 50.00 | 500.00 | ±105.00 |
| 75.00 | 750.00 | ±157.00 |
| 100.00 ← | 1,000.00 | ±210.00 |
| 125.00 | 1,250.00 | ±262.00 |
| 150.00 | 1,500.00 | ±315.00 |
| 200.00 | 2,000.00 | ±420.00 |
Lead Time Impact
| Lead Time | LTD | σ LTD | Relative |
|---|---|---|---|
| 3 days | 300.00 | ±203.00 | |
| 5 days | 500.00 | ±205.00 | |
| 7 days | 700.00 | ±207.00 | |
| 10 days ← | 1,000.00 | ±210.00 | |
| 14 days | 1,400.00 | ±214.00 | |
| 21 days | 2,100.00 | ±220.00 | |
| 30 days | 3,000.00 | ±228.00 |
Planning notes, formulas, and examples
About the Lead Time Demand Calculator
Lead time demand (LTD) is the expected quantity of inventory that will be consumed between placing a replenishment order and receiving it. It's calculated by multiplying average daily demand by the supplier lead time in days. This seemingly simple calculation is one of the most important numbers in inventory management because it determines the minimum stock level needed to avoid a stockout during the replenishment window.
Accurate lead time demand calculation requires reliable data on both demand rates and supplier lead times. When either input is uncertain, the variability must be accounted for through safety stock. Lead time demand forms the core of the reorder point formula: ROP = Lead Time Demand + Safety Stock.
This calculator helps you determine lead time demand under various scenarios, including variable demand and variable lead times. It also shows the standard deviation of LTD, which feeds directly into safety stock calculations for more sophisticated inventory management.
When This Page Helps
Lead time demand is the foundation of every reorder point calculation. If you underestimate LTD, you'll set reorder points too low and experience frequent stockouts. If you overestimate it, you'll carry excess safety stock and waste capital. Getting this number right has a direct, measurable impact on both service levels and inventory investment.
This calculator is particularly valuable when dealing with variable demand or unreliable suppliers, where the simple point estimate (average demand × average lead time) misses the variability that actually drives stockout risk. By incorporating that variability, you can set data-driven safety stock buffers instead of guessing.
How to Use the Inputs
- Enter your average daily demand in units.
- Input the supplier lead time in days.
- Optionally enter the standard deviation of daily demand for variability analysis.
- Optionally enter the standard deviation of lead time for supply uncertainty.
- Review the expected lead time demand and its variability.
- Check the demand and lead time scenario tables for sensitivity analysis.
- Use the LTD and standard deviation outputs as inputs for your reorder point and safety stock calculations.
Formula used
Lead Time Demand (LTD) = Average Daily Demand × Lead Time
Standard Deviation of LTD:
σ_LTD = √(LT × σ_d² + d² × σ_LT²)
Where:
• d = Average daily demand
• LT = Lead time in days
• σ_d = Standard deviation of daily demand
• σ_LT = Standard deviation of lead time
Reorder Point = LTD + Safety Stock
Safety Stock = Z × σ_LTDExample Calculation
Result: Lead Time Demand: 1,000 units (σ = 210 units)
Average demand of 100 units/day over a 10-day lead time gives an expected LTD of 1,000 units. The standard deviation accounts for both demand variability (σ_d = 20) and lead time variability (σ_LT = 2): σ_LTD = √(10 × 20² + 100² × 2²) = √(4,000 + 40,000) = √44,000 ≈ 210 units. For a 95% service level, safety stock = 1.65 × 210 = 346 units, giving a reorder point of 1,346 units.
Tips & Best Practices
- Use actual demand data from at least 3-6 months to calculate average daily demand and its standard deviation.
- Include ALL lead time components: order processing, manufacturing, shipping, customs, receiving, and quality inspection.
- When lead time varies significantly, use the standard deviation formula rather than just padding the average.
- Track supplier performance to get accurate lead time data — promised vs actual delivery dates.
- For seasonal products, calculate separate LTD values for peak and off-peak periods.
- Express demand in the same time unit as lead time (daily demand with lead time in days).
- The LTD standard deviation drives safety stock needs more than the expected LTD itself.
Components of Lead Time
Supplier lead time is rarely just shipping time. A comprehensive lead time includes: internal requisition and approval time, order transmission to supplier, supplier processing and manufacturing time, packaging and shipping preparation, transit time, customs clearance (international), receiving dock to warehouse floor time, and quality inspection. Each component adds days and variability, making accurate lead time measurement essential for reliable LTD calculations.
The Impact of LTD Variability
The standard deviation of lead time demand is often more important than the expected LTD itself for inventory planning. Two products might both have an expected LTD of 1,000 units, but if one has a standard deviation of 50 and the other has 300, their safety stock requirements are vastly different. High variability compounds the challenge because it comes from two sources (demand and lead time) that can both move unfavorably simultaneously.
Sources & Methodology
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Frequently Asked Questions
Lead time demand determines how much inventory you'll consume while waiting for a replenishment order to arrive. If you don't have at least this amount on hand when you place an order, you'll stock out before the order arrives. It's the minimum inventory bridge between placing an order and receiving it, making it the core input for reorder point calculations.
Calculate the standard deviation of your daily demand from historical data, then use the extended LTD formula: σ_LTD = √(LT × σ_d² + d² × σ_LT²). This gives you the variability of lead time demand, which you multiply by the Z-score for your target service level to get safety stock. Without accounting for variability, your reorder point only covers the average scenario.
Calculate the standard deviation of your lead time from order history (at least 10-20 orders). Enter this as lead time variability. When lead time is highly variable, it becomes the dominant driver of safety stock needs. Consider dual sourcing or negotiating guaranteed lead times to reduce this variability.
Match what your supplier uses. If they ship on business days only, use business days for lead time and business-day demand rates. If they ship every day (common for overseas freight), use calendar days. The key is consistency — demand rate and lead time must use the same unit. Many businesses default to calendar days for simplicity.
LTD tells you the expected demand during replenishment; safety stock covers the UNCERTAINTY around that expectation. Together they form the reorder point: ROP = LTD + Safety Stock. Think of LTD as covering the average case and safety stock as covering the worst-case scenarios that fall outside the average.
No, lead time demand is always zero or positive since both demand and lead time are non-negative. However, the actual demand during any specific lead time period can be much higher or lower than the expected LTD. The standard deviation of LTD quantifies this range, and safety stock provides protection against the upside risk (higher-than-expected demand).
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