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Safety Stock Calculator: Reorder Point and Carrying Cost by Service Level
Safety stock is the buffer you hold so demand surges and late deliveries do not turn into stockouts. Too little and you miss orders; too much and you tie up cash and space. The right amount depends on how variable demand and lead time are and on the service level you choose to target.
Enter average and standard deviation of daily demand, average and standard deviation of lead time in days, and a service level. The calculator converts the service level to a z value, then returns safety stock, reorder point and annual carrying cost at your own rate, with a table across service levels and a second scenario to compare. The example values are illustrative; replace them with yours.
Demand and lead time
Carrying cost
Scenario B (same demand)
Your results
Safety stock
343.5 units
z × √(LT × σd² + d² × σLT²), with z = 1.645 for 95% service.
Reorder point
1,243.5 units
Average demand over lead time (900) plus safety stock.
Annual carrying cost of safety stock
$1,030
Safety stock × unit cost × carrying rate.
Same inputs at other service levels
| Service | z | Safety stock | Reorder pt | Carrying $/yr |
|---|---|---|---|---|
| 90% | 1.282 | 267.6 | 1,167.6 | $803 |
| 95% | 1.645 | 343.5 | 1,243.5 | $1,030 |
| 97.5% | 1.960 | 409.3 | 1,309.3 | $1,228 |
| 99% | 2.326 | 485.8 | 1,385.8 | $1,457 |
| 99.9% | 3.090 | 645.3 | 1,545.3 | $1,936 |
Scenario B vs base
Safety stock 232.4 units (−111.1), reorder point 932.4 (−311.1). Carrying cost $697 per year (−$333).
Method: z is the inverse of the standard normal distribution at your service level (Acklam rational approximation, accurate to about 1e-9). It assumes demand and lead time are independent and roughly normal; intermittent or skewed demand needs a different approach.
Free template · Excel workbook
Working Capital Diagnostic Workbook
DSO, DIO, DPO and cash conversion cycle from your balances, with the cash value of a day and a lever-by-lever release plan.
Illustrative example values are prefilled; replace them with your own. Everything is calculated in your browser, and Performis does not store what you enter. Our analytics record which calculator was used and a few summary figures, such as a rounded result.
The formulas
- z from service level
- z = Φ⁻¹(service level), the one-sided inverse standard normal (for example 95% → 1.645, 99% → 2.326)
- Safety stock
- SS = z × √(LT × σd² + d² × σLT²)
- Safety stock, fixed lead time
- SS = z × σd × √LT (when σLT = 0)
- Reorder point
- ROP = d × LT + SS
- Annual carrying cost of safety stock
- SS × unit cost × annual carrying rate
How to interpret your result
See which source of variability dominates
The two terms under the square root show demand variability and lead time variability separately. If the lead time term is larger, working with suppliers on reliability can cut safety stock more than better forecasting can.
Notice the shape of the service-level curve
Moving from 95% to 99% raises z from 1.645 to 2.326, about 41% more safety stock for four points of service. The table shows that trade-off on your own inputs.
Price the buffer with your own carrying rate
Carrying cost is only as good as the rate you enter. Include capital, storage, handling, insurance and obsolescence, and use your own figure rather than a generic one.
Use scenario B to test a lever
Change the lead time, its variability or the service level and see the change in safety stock and cash. A shorter, more reliable lead time often matters more than a lower service target.
Check the inputs before trusting the output
Standard deviations should come from enough history to be stable and from the same time unit as the average. Outliers such as one-off promotions or shutdowns can distort them.
Frequently asked questions
How do you calculate safety stock?
With variable demand and lead time, safety stock = z × √(LT × σd² + d² × σLT²), where z comes from your target service level, LT is average lead time, σd is the standard deviation of daily demand, d is average daily demand and σLT is the standard deviation of lead time. With fixed lead time it reduces to z × σd × √LT.
Where does the z value come from?
It is the one-sided standard normal value for your service level, for example 1.645 for 95% and 2.326 for 99%. This calculator computes it with an inverse-normal approximation accurate to about nine decimal places, so you can enter any service level, not just the common ones.
What is the reorder point?
Reorder point = average demand × average lead time + safety stock. It is the on-hand-plus-on-order level at which you place a new order so stock covers expected demand during the lead time, with safety stock absorbing the variability.
What does service level mean here?
It is the cycle service level: the probability of not running out of stock during a replenishment cycle. It is not the same as fill rate, which is the share of units or lines shipped from stock.
When is this method a poor fit?
It assumes demand and lead time are independent and roughly normally distributed. Intermittent, lumpy or heavily skewed demand, strong trends and very short histories need a different approach, such as a demand-distribution or simulation method.
Is my data stored or sent anywhere?
No. The calculator runs entirely in your browser, collects no personal information, and does not save what you type.
Take it further with a template
Free template · Excel workbook
Working Capital Diagnostic Workbook
DSO, DIO, DPO and cash conversion cycle from your balances, with the cash value of a day and a lever-by-lever release plan.
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