Demand Variability
Use daily demand standard deviation when demand changes from day to day. Higher variability increases safety stock.
Safety Stock & Reorder Point
Estimate the buffer stock needed for uncertainty, calculate when replenishment should be triggered, and review the current stock position.
Browser-based calculation
Service-level planning
CSV export support
Why use this tool?
Decision Supported
Use this tool when demand or supplier lead time is uncertain and you need to decide how much buffer stock to hold and when to reorder.
What It Tells You
It estimates safety stock, lead time demand, reorder point, inventory position, and whether the current position suggests replenishment.
Example Input
Load the sample to see daily demand, demand variability, lead time variability, service level, and current stock position values.
Sample Output Interpretation
The result explains whether inventory is above or below the reorder point and how much protection coverage the current position provides.
1. Demand, Lead Time, and Stock Inputs
Enter average demand, variability, lead time, service level, and current stock position.
Data is processed locally in the browser and is not uploaded to a server.
Lead Time Variability
Use lead time standard deviation when supplier or transit time is inconsistent. If lead time is stable, enter zero.
Inventory Position
Inventory position equals on-hand stock plus on-order stock minus allocated or backordered stock.
About Safety Stock and Reorder Point
Safety stock is a buffer that protects against demand and lead time uncertainty. Reorder point is the inventory position at which replenishment should be triggered.
Used together, these measures help planners reduce stockout risk while avoiding excessive inventory. The result depends heavily on demand quality, lead time reliability, and the service level target.
How to Use This Tool
- Enter average daily demand for the item.
- Enter daily demand variability and lead time variability.
- Select the target service level.
- Enter on-hand, on-order, and allocated stock to calculate inventory position.
- Review whether the inventory position is above or below the reorder point.
Methodology, Assumptions, and Limitations
Formula Logic
Notation
d = average daily demand, sigma_d = daily demand standard deviation, L = average lead time, sigma_L = lead time standard deviation, z = service level z-score
Lead time demand
LTD = d × L
Safety stock
SS = z × sqrt((L × sigma_d2) + (d2 × sigma_L2))
Reorder point
ROP = LTD + SS
Inventory position
IP = on-hand + on-order - allocated
Required Inputs
The model needs average daily demand, average lead time, demand variability, lead time variability, and service level. Stock position inputs add action guidance.
Assumptions
The calculation assumes demand and lead time uncertainty can be approximated with standard deviation and a normal service-level z-score.
Limitations
Intermittent demand, supplier constraints, minimum order quantities, capacity limits, perishability, and correlated demand may require deeper inventory analysis.
Practical Planning Guidance
Use Real Variability
Safety stock is only as reliable as the variability inputs. Use recent demand and lead time data where possible.
Segment Service Levels
High-value or critical items may justify higher service levels. Low-priority items may not need the same protection.
Pair With EOQ
Safety stock and reorder point answer when to order. EOQ helps estimate how much to order.
Review Regularly
Update parameters when demand patterns, supplier reliability, lead times, or service expectations change.
FAQ
What is the difference between safety stock and reorder point?
Safety stock is the buffer for uncertainty. Reorder point is the trigger level that includes expected lead time demand plus safety stock.
Does a higher service level always make sense?
Not always. Higher service levels increase safety stock and holding cost. Use higher targets where stockouts are more costly or operationally risky.
What if lead time is stable?
Enter zero for lead time standard deviation. The safety stock will then reflect demand variability during the average lead time.
Is this the same as EOQ?
No. Safety stock and reorder point help decide when to replenish. EOQ helps decide how much to order.