MalakarConsultingThe reorder point (ROP) is the inventory level at which you place a new order so that stock arrives before you run out. Set it too low and you stock out. Set it too high and cash sits on a shelf. The formula is simple; the work is in getting the inputs right, especially lead time and demand variability.
This guide shows the formula, three worked examples of increasing realism, the service-level table you need, and the import-specific adjustments that catch small importers. The examples use made-up numbers and are meant to be reworked with your own data.
The basic formula
Reorder point = (average daily demand × lead time in days) + safety stock
The first term, demand during lead time, is what you expect to sell while you wait for the order. The second, safety stock, is a buffer against demand or lead time turning out worse than average.
Example 1, no safety stock. You sell 40 units a day on average and your supplier delivers in 15 days. Demand during lead time is 40 × 15 = 600 units. If you reorder at exactly 600 and everything goes to plan, the new stock arrives the day you run out. Any surprise causes a stockout, so this is the zero-buffer case.
Adding safety stock
When demand varies but lead time is reliable, use:
Safety stock = z × σd × √L
where z is the number of standard deviations for your target service level, σd is the standard deviation of daily demand, and L is lead time in days.
| Target cycle service level | z-score |
|---|---|
| 84% | 1.00 |
| 90% | 1.28 |
| 95% | 1.65 |
| 97.5% | 1.96 |
| 99% | 2.33 |
| 99.9% | 3.09 |
These are standard normal distribution values. The "cycle service level" is the probability of not stocking out during a replenishment cycle. It is not the same as fill rate, which measures the share of units filled.
Example 2, variable demand. Average demand is 40 units a day with a standard deviation of 12. Lead time is a reliable 15 days and you want a 95 percent service level (z = 1.65). Safety stock = 1.65 × 12 × √15 = 1.65 × 12 × 3.873 = about 76.7, round up to 77 units. Reorder point = 600 + 77 = 677 units.
When lead time also varies
Imports rarely arrive exactly on schedule. If both demand and lead time vary and you assume they are independent, use:
Safety stock = z × √( L × σd² + d² × σL² )
where d is average daily demand, L is average lead time in days and σL is the standard deviation of lead time in days.
Example 3. Average demand d = 40, σd = 12, average lead time L = 30 days (an overseas supplier, door to door), σL = 5 days, service level 95 percent (z = 1.65).
- L × σd² = 30 × 144 = 4,320
- d² × σL² = 1,600 × 25 = 40,000
- Sum = 44,320, square root = about 210.5
- Safety stock = 1.65 × 210.5 = about 347 units
- Reorder point = (40 × 30) + 347 = 1,200 + 347 = 1,547 units
Look at how the lead-time variability term dominates: 40,000 vs 4,320. Reducing lead-time variance often does more for inventory than forecasting better. If the same supplier tightened delivery to σL = 2 days, the second term drops to 1,600 × 4 = 6,400, the sum to 10,720, the root to about 103.5, and safety stock to about 171 units. That is 176 fewer units of buffer for the same service level.
Getting the inputs from your own data
- Average daily demand. Use actual sales or usage over a recent, representative period, excluding stockout days (which understate demand) and one-off spikes. Use weekly demand and weekly lead time if daily data is too noisy, and keep units consistent throughout.
- Standard deviation of demand. Calculate it from the same history, at the same time unit. Forecast error is the better measure if you have a forecast, since safety stock protects against error, not against predictable seasonality.
- Lead time. Measure from the day you place the order (or the day you decide to place it) to the day stock is available for sale, not to the day the container leaves. For imports include production, inland transport, port time, customs clearance, drayage and receiving. Track the actual for the last 10 to 20 orders and calculate the average and standard deviation.
- Service level. Set it by item importance. A high-margin item that halts a production line justifies 99 percent. A slow-moving accessory may justify 90 percent.
Import-specific adjustments
- Add the customs step to lead time. A hold, an exam or a documentation problem adds days. If your history includes such delays, they are in your standard deviation. If you are new, be more conservative.
- Adjust for seasonality and holidays. Chinese New Year and peak ocean season lengthen lead time predictably. Raise the reorder point for orders that will cross those periods rather than using an annual average.
- Account for order minimums and container fills. If a container makes economic sense at 3,000 units, your practical order quantity may be larger than the economic order quantity (see the cycle stock guide).
- Think about total cost. A cheaper overseas source with a 45-day lead time and high variability needs far more safety stock than a domestic supplier with a 7-day lead time. Include the carrying cost of that buffer when you level bids (see the RFQ and bid leveling guide).
Using and reviewing the reorder point
- Compute the reorder point per SKU and store it in your inventory system or a spreadsheet.
- Compare on-hand plus on-order (the inventory position), not just on-hand, to the reorder point. Otherwise you will place duplicate orders while one is in transit.
- Recalculate whenever demand, lead time or service target changes materially, and at least quarterly for fast movers.
- Track stockouts and excess. Repeated stockouts mean the reorder point is too low or the inputs are stale; a pile of unmoved stock means it is too high.
Limits of the formula
The safety stock formulas assume roughly normal demand and independent orders. Intermittent or lumpy demand, new products with no history, and items with strong trends do not fit well. For those, use a longer cover-based rule (weeks of supply), simulate from actual history, or accept manual judgment with a review date. Do not treat the output as more precise than the data behind it.
References
- Standard inventory-theory formulas for reorder point and safety stock under normal demand (see any operations management text, for example Silver, Pyke and Peterson, Inventory and Production Management in Supply Chains)
Reviewed September 21, 2026. Worked examples use illustrative numbers.
Frequently asked questions
What is the reorder point formula?
Reorder point equals average daily demand multiplied by lead time in days, plus safety stock. For example, 40 units a day, a 15-day lead time and 77 units of safety stock gives a reorder point of 677 units.
How do I calculate safety stock?
With variable demand and a fixed lead time, safety stock is z times the standard deviation of daily demand times the square root of lead time. If lead time also varies, use z times the square root of (lead time times demand variance plus demand squared times lead-time variance).
What service level should I use?
It depends on the cost of a stockout and the cost of holding stock. Many businesses use 95 percent for standard items and 99 percent or more for critical items, and lower for slow-moving items. The z-scores are 1.65 for 95 percent and 2.33 for 99 percent.
Should reorder point use on-hand stock or inventory position?
Use inventory position, which is on-hand plus on-order minus backorders. Comparing only on-hand stock to the reorder point can trigger duplicate orders when a shipment is already in transit.
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