What is safety stock?
Safety stock is the buffer inventory you hold on top of expected demand during lead time. Its job is to absorb the two things that go wrong between reorders: demand comes in higher than forecast, or replenishment takes longer than planned. Size it right and you hit your target service level without drowning in working capital.
The full safety stock formula
SS = Z × √( L × σd² + d² × σL² )
Z = service-level factor (from your target fill rate) · L = average lead time (days)
σd = std dev of daily demand · d = average daily demand · σL = std dev of lead time (days)
σd = std dev of daily demand · d = average daily demand · σL = std dev of lead time (days)
Most free calculators only use Z × σd × √L — they assume lead time is perfectly stable. The moment your supplier or production lead time varies, that term d² × σL² kicks in, and it is often the bigger driver. This tool includes it, so you don't silently under-buffer.
Turning a service level into Z
You don't need a Z-table. Enter the service level you want — the probability of not stocking out in a cycle — and the calculator converts it to the matching Z-score with the inverse normal distribution. 90% → 1.28, 95% → 1.64, 99% → 2.33. Notice how the last few points of service level cost the most buffer.
Frequently asked questions
What is safety stock?
Extra inventory held to protect against variability in demand and lead time, so you don't stock out before the next replenishment arrives.
What is the safety stock formula with Z-score?
SS = Z × √(L × σd² + d² × σL²), where Z is the service-level factor, L is average lead time, σd is the std dev of daily demand, d is average daily demand, and σL is the std dev of lead time.
How do I choose a service level?
It's the probability of not stocking out during a cycle. Common targets: 90% (Z=1.28), 95% (Z=1.64), 99% (Z=2.33). Higher service levels need disproportionately more stock.
Why account for lead-time variability?
If lead time swings, that uncertainty often drives more safety stock than demand variability. Demand-only calculators understate the buffer whenever lead time isn't perfectly stable.