Reorder point and safety stock calculator
This reorder point calculator tells you when to place your next order so stock arrives before you run out. It adds safety stock to the demand you expect during the supplier’s lead time, using a service level you choose (the chance of not stocking out) and how much your daily demand and lead time vary.
Enter your average daily demand, lead time and variability and it returns the reorder point (also called the reorder level), the safety stock, the days of stock you have left and whether it is time to order. It runs in your browser with no sign-up.
How to use the Reorder Point & Safety Stock Calculator
- Enter average daily demand in units and the lead time in days.
- Enter the standard deviation of daily demand. Use the spread of your recent daily sales; 0 means perfectly steady demand.
- If your supplier’s delivery time varies, add the lead-time standard deviation in days.
- Choose a service level. 95% is a common starting point; fast sellers often use 97.5–99%.
- Enter stock on hand and on order to see how long until you must reorder.
Reorder Point & Safety Stock Calculator features
Both safety stock forms
The simple Z × σd × √L formula, and the combined demand-and-lead-time variability form.
Service-level Z-scores
Pick from common levels or type your own; the Z-score is computed exactly, not looked up from a rounded table.
Stock position check
On hand plus on order compared with the reorder point, with the days left before you must act.
The reorder point and safety stock formulas
Reorder point = average daily demand × lead time + safety stock. With 40 units a day and a 10-day lead time, lead-time demand is 400 units.
Safety stock = Z × σd × √L when the lead time is fixed, where σd is the standard deviation of daily demand and L the lead time in days. With σd = 10: 1.645 × 10 × √10 = 52 units, so the reorder point is 452, rounded up to 453.
When lead time varies too, use safety stock = Z × √(L × σd² + d² × σL²), where σL is the standard deviation of lead time in days. The second term usually dominates for high-volume items: a lead time that swings by 2 days at 40 units a day adds 80 units of risk.
This model assumes demand is roughly normally distributed and independent from day to day. Seasonal spikes, promotions and very slow items break that assumption.
Service level and Z-score
Z is the point on the standard normal curve below which that share of outcomes falls. Each extra step in service level costs more safety stock than the last, so price the cost of a lost sale against the cost of holding units. To find the right order size, use the EOQ calculator.
| Service level | Z-score | Stock-out chance per cycle |
|---|---|---|
| 50% | 0.0000 | 50% |
| 80% | 0.8416 | 20% |
| 90% | 1.2816 | 10% |
| 95% | 1.6449 | 5% |
| 97.5% | 1.9600 | 2.5% |
| 99% | 2.3263 | 1% |
| 99.9% | 3.0902 | 0.1% |
Limits to keep in mind
- Standard deviations should come from your own sales history over a representative period, ideally 8–12 weeks or more.
- Service level here means the chance of no stock-out in a replenishment cycle, not the share of units filled (fill rate).
- If supplier lead time is usually quoted as a range, treat the midpoint as L and a quarter of the range as a rough σL.
- It is an inventory planning formula, not advice for any specific business.
Reorder Point & Safety Stock Calculator FAQ
How do I calculate the reorder point?
Multiply average daily demand by lead time in days, then add safety stock. For 40 units a day, a 10-day lead time and 52 units of safety stock, the reorder point is 452 units.
What is the safety stock formula?
Safety stock = Z × σd × √L for a fixed lead time, or Z × √(L × σd² + d² × σL²) when both demand and lead time vary.
What service level should I choose?
Choose by the cost of a stock-out. 90–95% suits steady, low-margin items; 98–99% suits best-sellers where a stock-out loses rank or customers. Higher levels need disproportionately more stock.
Is reorder level the same as reorder point?
Yes. Reorder level, reorder point and ROP all name the stock level that triggers a new purchase order.
Should on-order units count?
Yes. Compare the reorder point with stock on hand plus units already on order, otherwise you will reorder the same stock twice.
What if I do not know the standard deviation?
Calculate it from daily or weekly sales in a spreadsheet with STDEV. As a rough guide, many sellers start with a spread of 25–50% of average demand and refine it.