How to Calculate Reorder Points That Prevent Stockouts

Calculate precise reorder points using lead time, demand variability, and safety stock to prevent costly stockouts.

1. Calculate your average daily usage rate. Pull sales data for the last 90 days and divide total units sold by 90. If you sold 1,800 widgets in 90 days, your average daily usage is 20 units. Use 90 days minimum to capture seasonal variation — 30 days creates false precision.
2. Measure your supplier lead time accurately. Track actual delivery times from the last 10-15 orders, not quoted lead times. Calculate the average and note the longest delay. If orders typically arrive in 7 days but sometimes take 12, your average lead time is 7 days with a maximum observed of 12 days.
3. Calculate base lead time demand. Multiply average daily usage by average lead time. With 20 units daily usage and 7-day lead time, your base demand is 140 units. This covers normal conditions but ignores variability — the reason businesses stock out.
4. Determine safety stock using demand standard deviation. Calculate the standard deviation of daily sales over 90 days, then multiply by the square root of lead time days and your service level Z-score. For 99% service level (Z=2.33), 5-unit daily standard deviation, 7-day lead time: safety stock = 2.33 × 5 × √7 = 31 units.
5. Set your reorder point formula. Add lead time demand plus safety stock. Using the example: 140 units (base demand) + 31 units (safety stock) = 171 units reorder point. When inventory hits 171 units, place your next order.
6. Adjust for high-value or critical items. Increase safety stock for items with high stockout costs or unpredictable supply chains. For products generating $500+ daily margin, consider 99.5% service levels (Z=2.58). For commodity items with reliable suppliers, 95% service levels (Z=1.65) may suffice.