How to calculate safety stock
Enter the five values the calculator requests, then select Calculate Safety Stock. It calculates safety stock with demand variability, lead-time variability, and the target service level, then shows a reference reorder point.
Enter the calculator inputs
Use the same time basis throughout: demand in units per day and lead time in days. Enter:
- Average daily demand: typical units used or sold each day.
- Standard deviation of daily demand: daily demand variability, in units/day.
- Average lead time (days): typical days between placing an order and receiving it.
- Standard deviation of lead time (days): lead-time variability, in days.
- Target service level (%): enter a number such as
95for a 95% service level.
Calculate and review the result
Select Calculate Safety Stock. The calculator converts the service level to a Z-score and applies:
Safety stock = Z × √((Average lead time × Demand std dev²) + (Average demand² × Lead time std dev²))
The result shows safety stock, the Z-score, and a reference reorder point. It also lists the assumptions, variance terms, and combined standard deviation used. Displayed numbers are formatted for presentation.
Copy or download the result
- Select Copy result to copy the plain-text result. If clipboard access is unavailable, the interface tells you to select and copy it manually.
- Select Download result (.txt) to save the result as a text file.
- Select Reset to clear the inputs and hide the result.
Assumptions and limitations
Demand and lead time should be approximately normally distributed and independent. The calculator requires demand and lead time above zero, standard deviations of zero or more, a service level greater than 50 and less than 100, and some variability in either demand or lead time. It does not model ordering cost, holding cost, supplier constraints, or whether these statistical assumptions fit your inventory system. The result is not personalized inventory or purchasing advice.
Privacy and calculation behavior
The calculator script performs the calculation in the page from the values you enter. The inspected calculator implementation contains no data-submission call. This article does not make a broader claim about analytics or other site-wide data handling.
Frequently Asked Questions
What is safety stock?
Safety stock is the extra inventory held above expected demand during lead time, used as a buffer against variability in demand and lead time so you can meet a target service level without running out of stock.
What is the safety stock formula used here?
Safety stock = Z x square root of ((Average lead time x Demand standard deviation squared) + (Average demand squared x Lead time standard deviation squared)). Z is the value from the inverse normal distribution corresponding to your target service level, and this version accounts for variability in both demand and lead time.
How is the Z-score determined from the service level?
The calculator converts your target service level (for example 95%) into a Z-score using the inverse of the standard normal cumulative distribution function, the same method used in statistics textbooks and spreadsheet functions such as NORM.S.INV.
What units should demand and lead time use?
Demand and its standard deviation should be entered as units per day, and lead time and its standard deviation as days. Keep all inputs on the same daily basis - convert weekly or monthly figures to a consistent daily unit before entering them.
Does this tool fetch live rates or give personalized inventory advice?
No. It performs a single, transparent statistical calculation using only the numbers you type in. It does not fetch supplier data, market records, or live rates, and it is not personalized business or inventory advice.