# Safety Stock


Safety stock is extra inventory held as a buffer against demand spikes and supply delays, so a stockout does not happen the moment forecasts are wrong. It is sized from demand variability, lead time, and the service level a business wants to hit. More safety stock raises service but ties up cash.

## Why safety stock exists

Demand and lead time both vary. Safety stock absorbs that uncertainty so the business keeps filling orders when a shipment is late or a week sells faster than expected. It is the cushion that sits behind the reorder point.

## Sizing safety stock

Common formulas combine demand variability, lead-time variability, and a service-level factor. Hold too little and fill rate drops; hold too much and cash sits on the shelf. Accurate lead-time data, including order-processing time, makes the calculation tighter.

## Safety stock formula

The standard formula assumes lead time is fixed and daily demand varies. It multiplies a service-level factor by the standard deviation of daily demand and by the square root of the lead time.

```
Safety stock = Z × σd × √L

Z  = service-level factor (1.65 for a 95% service level)
σd = standard deviation of daily demand, in units
L  = lead time, in days
```

## When lead time varies too

If supplier lead time swings as much as demand does, use the combined formula. It adds the lead-time variability term, so an unreliable supplier raises safety stock even when demand is steady.

```
Safety stock = Z × √( L × σd² + D² × σL² )

D  = average daily demand, in units
σL = standard deviation of lead time, in days
```

## Worked example

A distributor sells an average of 120 cases a day of one item, with a standard deviation of 30 cases. Supplier lead time is 9 days and the target service level is 95%.

Safety stock = 1.65 × 30 × √9 = 1.65 × 30 × 3 = 148.5, so about 149 cases. The reorder point is average demand during the lead time plus safety stock: 120 × 9 + 149 = 1,229 cases. When on-hand stock falls to 1,229 cases, the buyer places the next purchase order.

## Service level factors

The Z value comes from the normal distribution and sets how often you accept a stockout during a replenishment cycle. Each step up in service level costs more inventory than the last.

| Target service level | Z factor |
| --- | --- |
| 80% | 0.84 |
| 85% | 1.04 |
| 90% | 1.28 |
| 95% | 1.65 |
| 97% | 1.88 |
| 98% | 2.05 |
| 99% | 2.33 |
| 99.9% | 3.09 |

## The max-minus-average shortcut

Teams without clean demand history often use a rule of thumb instead. It is easy to run from an ERP report, but it assumes the worst day and the worst lead time happen together, so it usually overstates the buffer.

```
Safety stock = (max daily sales × max lead time) − (average daily sales × average lead time)
```

## How order processing time changes safety stock

Lead time starts when the buyer sends the PO, not when the supplier keys it in. If a PO sits in an inbox for two days before anyone enters it, those two days are part of L. In the example above, cutting lead time from 9 days to 7 drops safety stock from 149 cases to 131 (1.65 × 30 × √7), 18 fewer cases of cash sitting on the shelf for one item. Same-day order entry shortens lead time for every item a customer buys.

## FAQ

### What is safety stock?

Safety stock is buffer inventory held to protect against demand and supply variability, so a late shipment or a demand spike does not immediately cause a stockout.

### How is safety stock calculated?

Safety stock is sized from demand variability, lead-time variability, and a target service level. Higher service levels and more variability require more safety stock.

### What is the difference between safety stock and reorder point?

Safety stock is the buffer itself. The reorder point is the stock level that triggers a new purchase order, calculated as average daily demand times lead time, plus safety stock.

### What service level should I use for safety stock?

Many distributors use 95% as a default and raise fast-moving or high-margin items to 98% or 99%. Slow, low-margin items often sit at 90%, because each step up in service level needs more inventory than the last.

### Is safety stock the same as buffer stock?

Yes. Safety stock, buffer stock, and buffer inventory all describe extra inventory held to protect against demand and supply variability.

## Sources

- [ASCM supply chain terminology](https://www.ascm.org)