# Safety Stock Calculator

URL: https://speedycalculator.com/calculators/finance/safety-stock/
Last updated: 2026-10-25

> The safety stock calculator works out how much extra inventory to keep to avoid stock-outs, with the basic max–average method or the statistical method based on a service level and the variability of demand and lead time, plus the reorder point.

## Inputs

- Method
- Maximum daily usage
- Maximum lead time (days)
- Average daily usage
- Average lead time (days)
- Service level
- Standard deviation of daily demand
- Standard deviation of lead time (days)

## Results

- Safety stock
- Reorder point
- Days of cover from safety stock

## Formula

```
Basic: safety stock = max daily usage × max lead time − average daily usage × average lead time
Statistical: SS = Z × √(LT × σ_d² + d² × σ_LT²)
Reorder point = average daily usage × average lead time + safety stock
```

## Explanation

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

FieldWhat to enterMethodChoose one: Basic (max and average), Statistical (service level)Maximum daily usageA numberMaximum lead timeIn daysAverage daily usageA numberAverage lead timeIn daysService levelChoose one: 90%, 95%, 97.5%, 99% …Standard deviation of daily demandA numberStandard deviation of lead timeIn 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 usageSafety stock (units)3040452206040075580907601201,120

## Worked 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.

## FAQ

**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.