Method 1
Demand variability only
59.2
SS = z × σd × √L = 1.6449 × 12 × √9 = 59.2
Safety Stock Calculator
Compare four common safety stock methods side by side, with an accurate z-score for any service level. Free, runs in your browser, nothing is uploaded.
Calculator
z for 95% = 1.6449
Method 2
Demand + lead time variability
Recommended when lead time varies
174.8
SS = z × √(L × σd² + d² × σL²) = 1.6449 × √(9 × 12² + 50² × 2²) = 174.8
Method 3
Lead time variability only
164.5
SS = z × d × σL = 1.6449 × 50 × 2 = 164.5
Enter max daily demand and max lead time to see method 4 (max minus average).
Reorder point (method 2)
624.8
ROP = d × L + SS = 50 × 9 + 174.8 = 624.8
Check whether it’s time to order →How it works
The z-score is the number of standard deviations of protection that gives your chosen service level, from the inverse of the standard normal distribution (95% → 1.6449).
Method 1 covers demand swings during a fixed lead time. Method 2 also covers late or early deliveries, combining both variances. Method 3 covers lead time swings only. Method 4 is a rule of thumb: the worst case minus the average case.
FAQ
What service level should I use?
Service level here is the probability of not stocking out during a replenishment cycle. 95% is a common starting point; use higher levels for critical or high-margin items and lower ones for low-value, easily substituted items.
Why are the four methods so different?
They protect against different risks. The demand-only method ignores late deliveries; the combined method adds lead time variability; max-minus-average is a simple rule of thumb that often overstates safety stock.
What units should I use?
Use the same time unit for demand and lead time. This calculator uses days.
Is my data uploaded?
No. Calculations and CSV parsing run in your browser, and the numbers and files you enter are never sent to a server, unless you choose to share a link, which carries the single-item inputs in its address. The page has no ads, sign-up or third-party trackers; the site host keeps standard anonymous visit counts.
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