Safety stock is the extra inventory held to cover uncertainty in demand and supplier lead time. This calculator uses the common statistical method: a service level factor multiplied by the variation in demand during the lead time.
Enter your numbers below. The reorder point is calculated alongside it.
Safety Stock Calculator
Enter your numbers to see the result.
Estimates for planning only. Results depend on the accuracy of the inputs and the assumptions of the standard formula.
How safety stock is calculated
When both demand and lead time vary, the usual formula is:
Safety stock = Z x square root of ( L x sigma_d squared + d squared x sigma_L squared )
- Z is the service level factor from the normal distribution (90% = 1.28, 95% = 1.645, 97.5% = 1.96, 99% = 2.33, 99.9% = 3.09).
- L is average lead time in days.
- d is average daily demand and sigma_d is its standard deviation.
- sigma_L is the standard deviation of lead time in days.
If lead time is fixed, the formula reduces to Z x sigma_d x square root of L. The reorder point is then d x L + safety stock.
Worked example
A product sells 100 units a day on average with a standard deviation of 20 units. Lead time is 9 days and does not vary. The target service level is 95%, so Z = 1.645.
- Safety stock = 1.645 x 20 x square root of 9 = 1.645 x 20 x 3 = 98.7, about 99 units
- Demand during lead time = 100 x 9 = 900 units
- Reorder point = 900 + 98.7 = about 999 units
Limits of the method
The formula assumes demand is roughly normally distributed and that past variation predicts the future. It works best for items with steady, frequent demand. For intermittent or highly seasonal items, or after a sudden change in supply, the result can be too low or too high, so review it against actual stockouts.
Raising the service level increases safety stock quickly: going from 95% to 99.9% nearly doubles the Z factor. Set targets by item class, with higher service for critical items and lower for slow, low-value ones.
Better forecasts and shorter, more reliable lead times lower the safety stock you need. Our demand planning guides cover forecast accuracy and replenishment.
Common questions
What service level should I choose?
It depends on the cost of a stockout compared with the cost of holding extra stock. Many businesses use 95% as a starting point, then set higher targets for critical or high-margin items and lower ones for slow, low-value items. Test the choice by comparing carrying cost with the cost of lost sales.
What if I do not know the standard deviation of demand?
Calculate it from recent history, for example daily or weekly sales over the last several months, using the standard deviation function in a spreadsheet. Use the same time unit for the average and the deviation, and exclude periods distorted by stockouts or one-off orders.
Does safety stock replace the reorder point?
No. Safety stock is the buffer. The reorder point is the stock level that triggers an order, and it equals expected demand during lead time plus the safety stock. Both are needed for a complete replenishment rule.
Why does lead time variation matter so much?
A late delivery leaves you selling at normal rates for extra days, so its effect grows with daily demand. The d squared x sigma_L squared term captures this. For fast-moving items with unreliable suppliers, lead time variation often outweighs demand variation.
How often should safety stock be recalculated?
Review it whenever demand patterns or supplier performance change, and on a regular schedule such as quarterly for important items. A buffer based on last year's variation can be wrong after a supplier change, a new channel or a shift in customer ordering habits.