The max-minus-average method in full, computed from your inputs. Safety stock is the gap between the most you could sell over the longest wait and the demand you would expect under normal conditions — the cushion that bridges the two.
Scenario
Calculation
Units of demand
Worst case
15/day × 10 days
150
Normal case
10/day × 7 days
70
Safety stock ◀
Worst case − normal case
80
Estimates only — not financial, tax, or professional advice.
Calculated in your browser — the numbers you enter are never sent to our servers.
What it calculates: Safety Stock, Max Demand During Lead, Average Demand During Lead.
Peak demand across the worst lead time, less the normal case
This uses the classic formula: maximum daily demand times maximum lead time, minus average daily demand times average lead time. It sizes the buffer to cover a cycle where both things go wrong at once — unusually strong sales and an unusually slow supplier.
Safety stock = (max daily demand × max lead time) − (average daily demand × average lead time)
Its appeal is that every input is a number you can read off your own history rather than a statistical parameter. Its cost is conservatism: assuming both worst cases coincide holds more stock than a probabilistic method would.
The buffer is insurance, and insurance has a premium
Safety stock ties up capital, consumes storage and carries obsolescence risk, and it earns nothing while it waits. The inventory carrying cost calculator prices that at typically 15% to 30% of the stock value a year.
So the question is not how to eliminate stockouts but what service level is worth paying for. Going from 95% to 99% availability often costs considerably more buffer than the first 95% did, because you are covering rarer and more extreme cases.
A predictable high-volume product needs less buffer than an erratic small one
Two products selling the same quantity need very different buffers if one sells steadily and the other arrives in unpredictable bursts. The formula captures this through the gap between average and maximum, which is wide for erratic demand and narrow for steady demand.
The same holds for suppliers. A reliable supplier with a consistent 14 days needs far less protection than one averaging 10 days but ranging from 5 to 30.
No service level, no seasonality, no correlation
This method does not take a target service level, so it cannot tell you the probability of a stockout it protects against. Statistical methods using demand standard deviation and a service factor do, and are worth the extra inputs for high-value lines.
It also assumes the maxima you enter remain valid. Seasonal peaks, a viral product moment or a supplier changing factories all invalidate a buffer computed from last year’s extremes.
Frequently Asked Questions
What is safety stock?
Safety stock is extra inventory held as a buffer so you do not run out when sales spike or a delivery is late. It absorbs the variability that average-based planning ignores, protecting your service level and preventing lost sales.
How is safety stock different from the reorder point?
Safety stock is the buffer itself. The reorder point is the trigger level that includes both expected demand during the lead time and that safety-stock buffer. You compute safety stock first, then add it when setting the reorder point.
Why does more variability mean more safety stock?
The bigger the gap between your worst case and your normal case, the more buffer you need to bridge it. Highly variable demand or unreliable lead times widen that gap, so the formula produces a larger safety stock to keep you protected.
Is the max-minus-average method accurate?
It is a practical approximation that works well when you have reliable estimates of your worst and normal cases. For tighter control, a statistical approach using the standard deviation of demand and a target service level gives a more calibrated buffer.
Can safety stock be zero?
If your maximum demand and lead time equal your averages, the formula returns zero, meaning no variability to buffer. In practice that is rare, and most sellers carry at least a small buffer because real demand and delivery times fluctuate.
Does carrying safety stock cost money?
Yes. Safety stock ties up capital and incurs storage, insurance, and obsolescence costs. The trade-off is between that carrying cost and the cost of stockouts, so the goal is enough buffer to stay reliable without overstocking.
Sources & References
Figures on this page are checked against primary, authoritative sources. Links open in a new tab.
Results are estimates for planning and analysis based on the figures you enter. They are not accounting, tax, or financial advice — verify with your own records and a qualified professional before making decisions.
Built and maintained by Jay Sudha, who runs a rice mill and trading business in Kashipur, India, and works with pricing, margins, working capital and bank credit as part of it.
Published the calculator with its formula, worked example, assumptions, limitations and a bespoke guide, and added an automated formula test suite covering it.
Tested the max-minus-average buffer against hand-computed peak and normal demand across their respective lead times.
Tested that identical maximum and average figures produce no buffer at all, and that the result is never negative.
Frequently Asked Questions now show on the page: 6 answers, checked in a page-by-page audit.
Add this calculator to your site
Responsive embed — and private: nothing your visitors type leaves their browser.