Reorder Point Calculator
Enter your average daily demand and supplier lead time to find the inventory level at which you should place a new order. Switch between the simple (max minus average) safety stock method and the statistical Z-score method that accounts for demand variability and your target service level. Results update instantly as you type.
What is a reorder point?
A reorder point (ROP) is the minimum quantity of a product you should have in stock before you need to place a new purchase order. When inventory falls to this threshold, it triggers an order so that the incoming stock arrives before you run out. The reorder point accounts for how quickly you sell the item (demand rate) and how long it takes the supplier to deliver (lead time), plus a safety buffer for unexpected demand spikes or delivery delays. Getting the reorder point right prevents both stockouts, which lose you sales, and overstock, which ties up cash and storage space.
Reorder point formula
The classic formula is: Reorder Point = (Average Daily Demand x Average Lead Time) + Safety Stock. The first part, average daily demand multiplied by lead time, tells you how many units you expect to sell while waiting for the new delivery. Safety stock is the extra buffer sitting below that. Without it, any demand spike or late delivery results in a stockout. The two most common ways to calculate safety stock are the simple method and the statistical method. The simple method uses (Maximum Daily Demand x Maximum Lead Time) minus (Average Daily Demand x Average Lead Time). The statistical method uses Z-score multiplied by the combined standard deviation of demand during lead time, which is sqrt(Lead Time x sigma_demand^2 + Demand^2 x sigma_leadtime^2). The statistical approach is more precise because it uses actual variability data rather than historical maximums.
Simple vs. statistical safety stock methods
The simple method compares your worst-case scenario (maximum demand over maximum lead time) against your average scenario. It is easy to apply with minimal data: you just need the maximum and average values for daily demand and lead time. Its weakness is that it tends to overestimate safety stock when the peaks you observed were one-off events, and it does not adjust for how often stockouts matter (service level). The statistical method uses the standard deviation of daily demand and optionally of lead time, combined with a Z-score that corresponds to your target service level. For example, a 95% service level uses Z = 1.65, meaning you will have enough stock during 95 out of 100 replenishment cycles. The statistical method sizes the buffer in proportion to actual variability rather than assuming the worst case every time. Both methods are correct; the best choice depends on how much demand history you have.
Carrying cost of safety stock
Safety stock is not free. Every unit sitting in your warehouse incurs holding costs: storage space, insurance, obsolescence risk, and the opportunity cost of the capital tied up. A standard estimate is 20 to 30 percent of unit value per year, with 25 percent as a common default. For example, 200 units of safety stock valued at $10 each would cost roughly $500 per year (200 x $10 x 25%). This calculator estimates this cost when you enter a unit cost, so you can compare the cost of carrying more safety stock against the cost of stockouts.
Service level Z-scores for safety stock
| Service Level | Z-Score | Interpretation |
|---|---|---|
| 80% | 0.84 | Stockout in 1 of 5 replenishment cycles |
| 85% | 1.04 | Stockout in 3 of 20 cycles |
| 90% | 1.28 | Stockout in 1 of 10 cycles |
| 95% | 1.65 | Common retail/wholesale target |
| 97% | 1.88 | High-value or critical items |
| 99% | 2.33 | Very high service requirement |
| 99.5% | 2.58 | Mission-critical parts or pharmaceuticals |
Standard normal Z-scores used in the statistical safety stock formula. Higher service levels require more safety stock.
Frequently asked questions
What is the basic reorder point formula?
Reorder Point = (Average Daily Demand x Average Lead Time) + Safety Stock. If your lead time is 7 days, you sell 50 units per day on average, and you carry 100 units of safety stock, your reorder point is (50 x 7) + 100 = 450 units. When stock drops to 450 units, place a new order.
How do I calculate safety stock?
The simple method is: Safety Stock = (Maximum Daily Demand x Maximum Lead Time) minus (Average Daily Demand x Average Lead Time). For example, if your max daily demand is 75 units and max lead time is 10 days, but averages are 50 units and 7 days, safety stock = (75 x 10) - (50 x 7) = 750 - 350 = 400 units. The statistical method uses Safety Stock = Z x sqrt(Lead Time x sigma_demand^2 + Demand^2 x sigma_leadtime^2), where Z is the Z-score for your target service level.
What is a service level in inventory management?
Service level is the probability of not having a stockout during a replenishment cycle. A 95% service level means that in 95 out of 100 order cycles you will have enough stock to meet all demand without running out. The higher the service level, the more safety stock you need. Most retailers target 90 to 95 percent for standard items and 99 percent or higher for critical or high-margin products.
What is a Z-score and how does it relate to safety stock?
A Z-score is a standard normal distribution factor that converts a desired probability (service level) into a multiplier for your demand variability. For a 95% service level, Z = 1.65, meaning you add 1.65 standard deviations of lead-time demand as safety stock. A higher Z-score demands more safety stock but raises your service level. The values are: 80% = 0.84, 90% = 1.28, 95% = 1.65, 99% = 2.33.
How is reorder point different from reorder quantity?
The reorder point tells you WHEN to order, specifically, at what inventory level you should place an order. The reorder quantity (or economic order quantity, EOQ) tells you HOW MUCH to order. They are separate decisions. Your reorder point could be 200 units, but you might order 500 units each time based on EOQ calculations that balance ordering costs against holding costs.
What happens if I set my reorder point too low?
If your reorder point is too low, your new stock will not arrive before you sell out. A stockout means lost sales, backorders, and dissatisfied customers. For a retailer, a single stockout event can cost more than a full year of safety stock carrying costs, which is why most businesses err slightly on the higher side for their reorder points.
Do I need safety stock if my demand and lead time are constant?
Technically no. If demand is perfectly steady at exactly the same rate every day and your supplier always delivers in exactly the same number of days, you can set the reorder point to exactly Average Daily Demand x Lead Time with no safety stock. In practice, almost every business has some variability in both, so some safety stock is almost always worthwhile.