Running out of stock is one of the fastest ways to lose customers, yet overstocking ties up cash that your business could use elsewhere. The reorder point is the inventory level at which you should place a new purchase order so that new stock arrives before you run out — and this calculator builds it from average demand, lead time, demand variability, and your chosen service level.
Enter your numbers below and the result card splits the answer into two parts: the average demand you expect during the supplier lead time, and the safety stock you hold as a buffer against unexpected spikes. Add them together and you have the reorder point in units.
How to Use
- Enter Average Daily Demand — the number of units you sell or consume per day on average. Use a 30 to 90 day history, not a single busy week.
- Set Daily Demand Std Dev — how much daily demand actually bounces around that average. A steady grocery staple might have a std dev of 2 units against a mean of 50; a seasonal gift item might see 20.
- Type Lead Time in days — the total clock time from hitting "send" on the PO to goods sitting on your shelf. Include supplier processing, transit, customs, and receiving inspection, not just the shipping label.
- Choose a Service Level Z-Score. Leave the default 1.65 for a 95% service level (you run out roughly 5% of cycles). Raise it to 1.96 for 97.5% or 2.33 for 99% on critical SKUs; drop to 1.28 for 90% on slow-movers where stock is cheap to hold.
- Read the result card. The Reorder Point line tells you the exact unit count at which to fire the next PO. The Safety Stock line is your buffer; if it feels large, either tighten the Z-score or negotiate a shorter lead time with your supplier.
The formula behind the calculator is the standard continuous-review (Q, R) reorder point taught in operations research textbooks and used by buyers at Amazon, Walmart, and every ERP system from SAP to NetSuite.
The Reorder Point Formula
The classic formula is:
ROP = (Average Daily Demand × Lead Time in Days) + Safety Stock
And safety stock itself is built from demand volatility and your service-level target:
Safety Stock = Z × σ(daily demand) × √(Lead Time in Days)
Where:
- ROP is the unit count that triggers a new purchase order.
- Average Daily Demand is your mean units sold per day.
- Lead Time is the replenishment cycle length in days.
- Z is the standard-normal score for your desired service level (1.28, 1.65, 1.96, 2.33 are the usual choices).
- σ(daily demand) is the standard deviation of daily demand over your history window.
Notice the square root on lead time. Variability grows with the square root of time, not linearly with time, because daily shocks partially cancel each other out. Doubling lead time only adds about 41% more safety stock (√2 ≈ 1.41), which is why even modest lead-time compression pays off disproportionately.
Worked example on default values: average demand 50 units/day, std dev 10, lead time 7 days, Z = 1.65. Lead-time demand is 50 × 7 = 350 units. Safety stock is 1.65 × 10 × √7 ≈ 1.65 × 10 × 2.646 ≈ 43.7 units. The reorder point is therefore 394 units — when on-hand inventory drops to 394, place the next order.
How to Pick the Right Z-Score
The Z-score is the single most leveraged number in the calculator. It translates directly into how often you accept a stockout:
- Z = 0.84 → 80% service level. You stock out in roughly one in five replenishment cycles. Fine for cheap, abundant, long-lived SKUs.
- Z = 1.28 → 90% service level. A common default for C-class items in ABC analysis.
- Z = 1.65 → 95% service level. The industry default for B-class and most retail SKUs.
- Z = 1.96 → 97.5% service level. Use for A-class fast-movers where a stockout costs a lost customer.
- Z = 2.33 → 99% service level. Reserve for mission-critical parts, hospital supplies, or SKUs with no back-up supplier.
Raising Z from 1.65 to 2.33 increases safety stock by about 41% on the same demand and lead time. That extra inventory ties up cash and inflates holding cost — only pay for it where the stockout cost genuinely justifies it.
Examples
Example 1 — A typical retail SKU. A boutique sells 50 candles per day on average, with a standard deviation of 10. The supplier takes 7 days to deliver, and the buyer wants a 95% service level. Lead-time demand is 350 units; safety stock is about 44 units; the reorder point lands at 394 candles. When stock falls below 394, place the next PO.
Example 2 — Long lead time, volatile demand. An auto parts shop sells 200 brake pads per day (std dev 40) but the overseas supplier takes 30 days. At Z = 1.96, lead-time demand is 6,000 units and safety stock is 1.96 × 40 × √30 ≈ 430 units. The reorder point is about 6,430 pads. Notice how the long lead time dominates the calculation — this is exactly where air freight or a local backup supplier pays off.
Example 3 — Large-volume warehouse. A distributor moves 200,000 units of a CPG staple per day (std dev 5,000) with a 14-day lead time and a 99% service level (Z = 2.33). Lead-time demand is 2,800,000 units; safety stock is 2.33 × 5,000 × √14 ≈ 43,590 units. The reorder point is roughly 2,843,590 units. The calculator handles volumes this large without rounding errors.
Example 4 — Zero variability. If demand is perfectly flat (std dev = 0), safety stock collapses to zero and ROP is simply average demand times lead time — 350 units in the default scenario. That is the mathematical answer when demand is 100% predictable; in the real world, even "stable" SKUs show some noise, so a small std dev is usually realistic.
FAQ
What is a reorder point?
The reorder point (ROP) is the on-hand inventory level at which you trigger a new purchase order. It is calculated as average demand during lead time plus safety stock, so that new stock arrives just before you would otherwise run out.
What is the difference between reorder point and reorder quantity?
The reorder point tells you when to order (the unit threshold). The reorder quantity — often set by the Economic Order Quantity, or EOQ — tells you how much to order each time. They are two halves of the continuous-review (Q, R) inventory policy.
How much safety stock should I carry?
Use the formula safety stock = Z × σ(daily demand) × √(lead time). Choose Z based on your service-level target: 1.28 for 90%, 1.65 for 95%, 1.96 for 97.5%, and 2.33 for 99%. Higher Z means fewer stockouts but more capital tied up in inventory.
Does the reorder point change with order size?
No. Under the standard continuous-review model, the reorder point depends only on demand, lead time, and service level — not on how many units you buy each cycle. Order size (EOQ) is a separate decision.
What if lead time itself is variable?
This calculator assumes a fixed lead time. When supplier lead times vary materially, replace σ(daily demand) with the combined variance: σ_combined = √((σ_demand × leadTime)² + (averageDailyDemand × σ_leadTime)²). Most ERPs apply this extended formula; the inputs on this page can be combined manually if your lead time swings widely.
Can I use this formula for raw materials or MRO items?
Yes. The same ROP formula works for finished goods, raw materials, components, and maintenance supplies, as long as you can estimate average daily usage, its standard deviation, and the replenishment lead time. For work-order-driven MRO, treat usage as demand.
What service level should I target?
Start with 95% (Z = 1.65) for most SKUs. Raise to 99% (Z = 2.33) on A-class items where a stockout loses a long-term customer, and drop to 90% (Z = 1.28) on slow-moving or expensive-to-hold items. Use ABC classification to tier Z-scores across your catalog rather than applying one number to everything.
Disclaimer
This reorder point calculator is provided for informational purposes only and does not constitute inventory, supply-chain, financial, or legal advice. Results are estimates based on the inputs you enter and the standard continuous-review ROP formula; actual stockout risk depends on demand distribution shape, supplier reliability, quality defects, returns, and other factors not modeled here. Always validate thresholds against your own ERP data and a qualified inventory planner before changing replenishment policy.