How to Calculate Reorder Points and Safety Stock
The Basic Reorder Point Formula
The reorder point formula at its simplest is: reorder point equals average daily demand multiplied by lead time in days, plus safety stock. If you sell 8 units per day of a product and your supplier takes 21 days to deliver, the demand during lead time is 168 units. Add safety stock (let us say 50 units for this example), and your reorder point is 218 units. When your on-hand quantity hits 218, you place the order. Over the next 21 days, you sell approximately 168 units, leaving you with 50 units of safety stock when the new shipment arrives. If demand was slightly higher than average or the supplier delivered a day late, the safety stock absorbed the difference.
Each component of the formula requires careful measurement. Average daily demand should come from at least 30 days of recent sales data, preferably 60 to 90 days. Shorter windows are more responsive to recent trends but more volatile. Longer windows are more stable but slower to capture demand shifts. For products with strong growth trends, weight more recent data more heavily or use the last 30 days exclusively, because a 90-day average that includes your early, slow-selling months understates current demand.
Lead time is the total elapsed time from when you send the purchase order to when the product is received, counted, and available to sell. This includes manufacturing or processing time at the supplier, transit time (ocean freight typically adds 25 to 35 days from China, air freight 3 to 7 days), customs clearance (1 to 7 days depending on the port and time of year), inland transportation to your warehouse, and your own receiving and inspection process. For domestic suppliers using ground shipping, total lead time is typically 3 to 14 days. For overseas suppliers, 60 to 120 days is common. Measure actual lead times over multiple orders, not just the supplier's quoted delivery time, because real-world performance almost always has more variability than what is promised.
Calculating Safety Stock
Safety stock is extra inventory beyond what you expect to sell during lead time, carried specifically to absorb unpredictable demand increases and supplier delivery delays. The simplest safety stock method is a fixed number of days of supply: if you want 7 days of protection at an average daily demand of 8 units, your safety stock is 56 units. This method works for products with relatively stable demand and reliable suppliers, and it is the easiest to calculate and explain.
A more precise method accounts for both demand variability and lead time variability using standard deviations. The formula is: safety stock equals Z score multiplied by the square root of (lead time in days multiplied by the standard deviation of daily demand squared, plus average daily demand squared multiplied by the standard deviation of lead time in days squared). The Z score corresponds to your desired service level: 1.28 for 90% (stockout protection 90% of the time), 1.65 for 95%, 2.33 for 99%. For most ecommerce products, a 95% service level strikes the right balance between protection and carrying cost. Only your absolute top sellers, where a stockout has severe consequences like losing an Amazon ranking, justify the cost of 99% service levels.
To put real numbers on this: suppose a product sells an average of 10 units per day with a standard deviation of 3 units, and your supplier's lead time averages 30 days with a standard deviation of 5 days. At a 95% service level (Z = 1.65), the safety stock calculation is 1.65 multiplied by the square root of (30 times 9, plus 100 times 25), which equals 1.65 times the square root of (270 plus 2500), which equals 1.65 times 52.6, which equals approximately 87 units. Your total reorder point would be (10 times 30) plus 87, equaling 387 units. Compare that to the simple method: 10 times 30 plus 7 days times 10 equals 370 units. The statistical method is higher because it accounts for the significant variability in your supplier's lead time.
Reorder Point Examples by Business Type
Domestic Supplier, Stable Demand
A seller buying printed t-shirts from a US-based print-on-demand supplier with 5-day lead time. Average daily sales: 12 units. Demand variability is low (standard deviation of 2 units). Lead time is consistent (standard deviation of 1 day). At a 95% service level, safety stock is approximately 17 units. Reorder point: (12 times 5) plus 17 equals 77 units. This seller places frequent, small orders because the short lead time allows it, keeping average inventory levels low and cash flow healthy.
Overseas Supplier, Variable Demand
A seller importing kitchen gadgets from China with an 80-day total lead time. Average daily sales: 6 units. Demand variability is moderate (standard deviation of 3 units). Lead time variability is high (standard deviation of 15 days). At a 95% service level, safety stock is approximately 165 units. Reorder point: (6 times 80) plus 165 equals 645 units. This seller carries substantial safety stock because both demand and lead time are unpredictable. The 165-unit safety buffer costs money to store but prevents stockouts that would take 80+ days to recover from.
Amazon FBA Seller
An Amazon FBA seller needs to factor in not just the manufacturing and shipping lead time but also Amazon's receiving time, which can range from 3 to 21 days depending on warehouse capacity and time of year. During Q4 (October through December), Amazon's inbound receiving can take 2 to 3 weeks, effectively adding 14 to 21 days to your lead time. FBA sellers should calculate two sets of reorder points: one for normal periods and one for peak season that accounts for longer receiving windows. Running out of stock on Amazon is particularly costly because your organic search ranking drops, your pay-per-click advertising history resets, and competitors take the sales you would have earned.
Common Reorder Point Mistakes
The most frequent mistake is using the supplier's quoted lead time instead of measuring actual lead time from your own order history. If your supplier says 30 days and your last 5 orders took 32, 35, 28, 41, and 33 days, your average actual lead time is 34 days with a standard deviation of 4.7 days. Using 30 days in your reorder point calculation would cause you to order 4 days too late on average, which at 10 units per day means 40 units of unplanned stockout risk on every order cycle.
Another common mistake is using the same reorder point for all products. Your reorder point should reflect the specific demand rate, lead time, and variability for each individual SKU. A product that sells 50 units per day with a 10-day lead time needs a completely different reorder point than a product that sells 2 units per day with a 90-day lead time, even though both might have similar total sales volume over a year. ABC analysis helps you prioritize which products deserve the most precise reorder point calculations.
Failing to update reorder points as conditions change causes gradual drift. If demand for a product grows 10% month over month, a reorder point calculated 6 months ago understates current demand by 60% or more. Review reorder points quarterly for your B and C items and monthly for your A items. Some inventory management software automatically recalculates reorder points based on rolling demand data, which eliminates this maintenance burden.
Setting Up Reorder Alerts
Once you have calculated reorder points, configure them as alerts in your sales platform or inventory software. In Shopify, you can set low stock notifications per product under the inventory section of each product page. When stock reaches the threshold, Shopify sends an email notification to your admin account. In WooCommerce, set the low stock threshold under each product's inventory tab. Most dedicated inventory platforms like Cin7, Ordoro, and Zoho Inventory support per-SKU reorder points with automatic purchase order generation, where the system not only alerts you but creates a draft PO with the suggested order quantity based on your demand forecast.
The suggested reorder quantity (how much to order, not just when to order) is a separate calculation. The simplest approach is to order enough to last until the next reorder cycle plus safety stock. If your reorder point triggers at 387 units, your average demand during lead time is 300 units, and you want 87 units of safety stock, you might order 400 units to bring your stock back to a target level of 787 (387 reorder point plus 400 units incoming). The Economic Order Quantity (EOQ) model provides a more precise answer by balancing ordering costs against holding costs, but for most ecommerce sellers, ordering to a target stock level based on forecasted demand over the next lead time period is practical and effective.