Reorder Point Calculator
The reorder point answers a narrow question: how low can stock fall before you must order again to avoid running out while the delivery is in transit? It has two parts — the demand you expect during the lead time, and a buffer for the demand you did not expect. The second part behaves in a way that surprises people: it grows with the square root of lead time, so doubling the lead time raises it by about 41%, not 100%.
What this generator does
Multiplies daily demand by the lead time to get expected demand during replenishment, then adds safety stock: the service-level z-score times the daily standard deviation times the square root of the lead time. It reports each part separately so the buffer can be seen for what it is.
How to use this tool
- Generate a scenario, or substitute your own daily demand, variability and lead time.
- Read the reorder point and note how it splits into expected demand plus safety stock.
- Compare service levels — 99% costs considerably more buffer than 95%.
- Try quadrupling the lead time and watch safety stock only double.
Understanding the controls
- Generate
- Produces a scenario with plausible daily demand, day-to-day variation, lead time and one of four common service levels.
- Seed
- Any word reproduces the same scenario, so a worked example can be set and marked later.
Common use cases
- Setting reorder levels for a stocked item with variable demand
- Teaching the square-root relationship between lead time and safety stock
- Showing what a higher service level actually costs in stock
- Worked examples for an inventory management course
- Producing practice problems with known answers
How this generator works
Demand over a lead time of several days is the sum of several days' demand, and the variance of a sum of independent draws adds — so the standard deviation grows with the square root, not in proportion. The z-score comes from inverting the normal distribution with a rational approximation accurate to several decimal places, checked in the tests against the familiar values of 1.645 at 95% and 2.326 at 99%.
Randomness and fairness
The scenario figures — daily demand, its variation, lead time and service level — are drawn randomly so the relationships can be explored. The reorder point is then a deterministic calculation. A seed reproduces the same scenario exactly and is therefore explicitly not cryptographically secure; without one the browser's cryptographically secure generator supplies the figures.
For how randomness is produced across the whole site, see how Generate Random works.
Limitations and good to know
- Assumes demand is normally distributed and independent between days; correlated demand needs a larger buffer.
- Lead time is treated as fixed. Variable lead time adds a second term to the safety stock formula.
- Service level here is the probability of not running out during a cycle, not the fraction of demand met.
- No stockout cost is modelled, so the service level is your choice rather than an optimised value.
- The page keeps no history, so seed a case you want to compare against later.
Privacy and your data
Every figure here is worked out on your own device. Neither the scenario nor your seed leaves the browser or is stored anywhere.
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