How to calculate safety stock
Enter the five values that describe demand, lead time, and your target service level. After you select Calculate Safety Stock, the page shows the Z-score, the two variance terms, safety stock, and a reference reorder point calculated from those inputs.
Use the safety stock calculator
- Enter Average daily demand and Standard deviation of daily demand in units per day.
- Enter Average lead time (days) and Standard deviation of lead time (days).
- Enter Target service level (%) as a number greater than 50 and less than 100, such as
95. - Select Calculate Safety Stock. The result section then displays the calculation.
- Review the values, then select Copy result, Download result (.txt), or Reset.
Safety stock formula for variable demand and lead time
The calculation converts Target service level (%) into a Z-score with the inverse standard normal distribution. It combines the demand variance term with the lead-time variance term, then multiplies the square root of their sum by Z.
Safety stock = Z x sqrt((Average lead time x Demand standard deviation^2) + (Average daily demand^2 x Lead time standard deviation^2))
Average daily demand: 40 units/day
Standard deviation of daily demand: 8 units/day
Average lead time (days): 7
Standard deviation of lead time (days): 2
Target service level (%): 95
Safety stock: 136.13 units
Reference reorder point: 416.13 units
Z-score for 95% service level: 1.645The result lists each entered value, both variance terms, the combined standard deviation, average usage during lead time, and the reference reorder point. The page rounds displayed values for presentation.
Safety stock input rules and assumptions
Average daily demand and Average lead time (days) must be greater than zero. Both standard deviation fields accept zero, but they cannot both be zero because the calculation requires demand or lead-time variability. Target service level (%) must be greater than 50 and less than 100. Keep demand and its standard deviation in units per day, and keep lead time and its standard deviation in days.
The method assumes that demand and lead time are approximately normally distributed and independent. It does not include ordering cost, holding cost, supplier constraints, or a check of those assumptions against your inventory system.
Where this safety stock calculation comes up
A warehouse planner can enter a SKU's average daily demand, demand variation, supplier lead time, lead-time variation, and target service level to calculate a buffer quantity and review the reference reorder point.
To compare supply scenarios, change the lead-time inputs while keeping demand constant. The result shows how the lead-time variance term changes. Select Download result (.txt) to save the entered assumptions and calculated values as a text file.
When to use a different inventory calculator
Use this page when demand and lead-time variability both belong in the safety stock calculation. For the wider set of Business Calculators, choose the Reorder Point Calculator when you have average daily demand, maximum daily demand, and lead time. Choose the Economic Order Quantity Calculator when your inputs are demand, ordering cost, and holding cost. Choose the Inventory Planning Calculator when you need safety stock with reorder point, EOQ, and inventory turnover calculations.
Frequently asked questions
How do I calculate safety stock from demand and lead time variability?
Enter Average daily demand, Standard deviation of daily demand, Average lead time (days), Standard deviation of lead time (days), and Target service level (%). Select Calculate Safety Stock. The result combines both variance terms and applies the Z-score for the service level.
What safety stock formula does this calculator use?
The calculator uses Z multiplied by the square root of the demand variance term plus the lead-time variance term. Z comes from the inverse standard normal distribution for your Target service level (%).
What service level can I enter for safety stock?
Enter a number greater than 50 and less than 100, such as 95 for a 95% service level. Values at or below 50, values at or above 100, blank values, and non-numeric values produce an error.
What is the difference between safety stock and a reorder point?
Safety stock is the variability buffer, while the reference reorder point adds average usage during lead time to that buffer. This page shows both values after the calculation.
Does the safety stock calculator use live inventory data?
The calculator performs the calculation from the numbers you enter. Its script contains no data-submission call for those values, and it does not retrieve supplier or inventory records.