Inventory planning
Safety stock calculator.
Pick a service level, enter demand and lead-time variability, and get the z-score, safety stock and reorder point.
Safety stock = z x sqrt(L x s_d² + d² x s_L²). With a fixed lead time this reduces to z x s_d x sqrt(L).
Your numbers
Result
Worked example
Service level 95%, average demand 10 units a day, daily demand standard deviation 3, lead time 14 days, no lead-time variability. z for 95% is 1.645. Safety stock = 1.645 x 3 x sqrt(14) = 18.5 units. Lead-time demand is 10 x 14 = 140, so the reorder point is about 158.5 units. If lead time itself varies with a standard deviation of 3 days, safety stock = 1.645 x sqrt(14 x 3² + 10² x 3²) = 1.645 x sqrt(1,026) = 52.7 units.
How it works
Safety stock covers variation during the replenishment lead time. The Wikipedia safety stock article gives, for uncertain demand and lead time, SS = z x sqrt(E(L) x s_D² + E(D)² x s_L²), where E(L) and s_L are the mean and standard deviation of lead time and E(D) and s_D those of demand per period. With a constant lead time it gives ROP = L x E(D) + z x s_D x sqrt(L).
z comes from the service level. The calculator inverts the standard normal distribution at your cycle service level (95% gives z = 1.645, matching the critical value in the NIST standard normal table). Cycle service level is the probability of no stockout during one replenishment cycle (Wikipedia, Service level), not the share of units filled.
Demand and lead time must use the same time unit. Demand is assumed roughly normal and independent from day to day, and demand and lead time are assumed independent. Spikes, promotions, MOQs and supplier delays outside your history are not modelled.
Sources (opened 2026-10-18): Safety stock - Wikipedia; Service level - Wikipedia; NIST/SEMATECH e-Handbook: Cumulative Distribution Function of the Standard Normal Distribution. The formula wording above follows these pages; the example figures and all arithmetic are ours.
Inputs are user-entered figures. Results are planning estimates, not financial advice or a forecast. Everything runs in your browser. Nothing you type is sent or stored.
FAQ
How do you calculate safety stock?
Multiply the z-score for your service level by the standard deviation of demand during the lead time. With a fixed lead time that is z x s_d x sqrt(L). If lead time also varies, use z x sqrt(L x s_d² + d² x s_L²).
What z-score do I use for a 95% service level?
About 1.645. 90% is about 1.282, 97.5% about 1.960 and 99% about 2.326. The calculator computes z for any level from 50% to just under 100%.
Why not 100% service level?
On a normal distribution a 100% cycle service level needs infinite safety stock, so the calculator accepts values below 100%.
Is safety stock the same as reorder point?
No. The reorder point is lead-time demand plus safety stock. Safety stock is only the buffer on top of expected demand during the lead time.