What this calculator does
The tool estimates a buffer for demand variation over a fixed replenishment lead time. It scales daily demand standard deviation by the square root of lead time and a service factor selected by the inventory policy.
When to use it
Use it when you have enough demand history to estimate variation and lead time is reasonably stable. It supplies the safety-stock term for the reorder point calculation.
Inputs and formula
z is the service factor; σ is standard deviation of daily demand; L is lead time in days. Safety stock = z × σ × √L
Worked example and interpretation
A service factor of 1.65, daily standard deviation of 4, and 21 days lead time returns about 30 units. Round according to practical packing and issue units, then test the actual stockout experience.
Practical notes and limitations
This formulation assumes fixed lead time and roughly normal, independent demand variation. Intermittent MRO demand, supply disruption, obsolescence, shelf-life constraints, and critical safety function can invalidate the approximation. Apply a criticality review alongside the math.