Safety stock buffers against uncertainty in demand and supplier lead times. A higher service level (the chance of not running out) needs disproportionately more stock: going from 95% to 99% raises Z from 1.65 to 2.33.
How to use the Safety Stock Calculator
- Method
- Maximum daily usage
- Maximum lead time
- Average daily usage
- Average lead time
- Service level
- Standard deviation of daily demand
- Standard deviation of lead time
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose a method, then enter usage and lead-time figures (and their standard deviations for the statistical method).
Tips
- Use figures for the same period (a month, a quarter or a year) for every input so ratios compare like with like.
- Track the metric over time – the trend usually tells you more than a single value.
- Compare your result with industry benchmarks, which vary a lot between sectors.
- Treat the result as a starting point for a decision, then check the assumptions behind each input.
Common mistakes to avoid
- Mixing monthly and yearly numbers in the same calculation.
- Using revenue where the formula needs profit (or the other way round).
- Forgetting hidden costs such as taxes, fees, overheads or your own time.
Inputs explained
| Field | What to enter |
|---|---|
| Method | Choose one: Basic (max and average), Statistical (service level) |
| Maximum daily usage | A number |
| Maximum lead time | In days |
| Average daily usage | A number |
| Average lead time | In days |
| Service level | Choose one: 90%, 95%, 97.5%, 99% … |
| Standard deviation of daily demand | A number |
| Standard deviation of lead time | In days |
What each result means
- Reorder point = [Average daily usage]×[Average lead time] + [Safety stock]
- Days of cover from safety stock = [Safety stock]÷[Average daily usage]
Example values
How the main result changes when Maximum daily usage changes and everything else stays at the example values above:
| Maximum daily usage | Safety stock (units) |
|---|---|
| 30 | 40 |
| 45 | 220 |
| 60 | 400 |
| 75 | 580 |
| 90 | 760 |
| 120 | 1,120 |
Formula
Basic: safety stock = max daily usage × max lead time − average daily usage × average lead timeStatistical: SS = Z × √(LT × σ_d² + d² × σ_LT²)Reorder point = average daily usage × average lead time + safety stockWorked example
Inputs — Method: Basic (max and average); Maximum daily usage: 60; Maximum lead time: 12 days; Average daily usage: 40; Average lead time: 8 days; Service level: 95%; Standard deviation of daily demand: 10; Standard deviation of lead time: 2 days.
Results — Safety stock = 400 units; Reorder point = 720 units; Days of cover from safety stock = 10 days.
Frequently asked questions
What is safety stock?
Extra inventory kept to cover unexpected demand or supplier delays.
How do I choose a service level?
Balance the cost of holding stock against the cost of running out; 95% is common.
What is the reorder point?
The stock level at which you place a new order.
What formula does the Safety Stock Calculator use?
Basic: safety stock = max daily usage × max lead time − average daily usage × average lead time.