What is safety stock?

Extra inventory held to cover demand or lead-time variability.

What is the safety stock formula?

Statistical: Z × standard deviation of demand × √lead time. Simple: max usage over max lead time minus average usage over average lead time.

The factor for a service level, such as 1.65 for 95%.

Should all items have the same service level?

No. Critical and A-class items usually justify a higher level.

Safety Stock Formula: How Much Buffer Stock Do You Need?

Safety stock is the extra inventory that protects you when demand rises or the supplier is late. Too little means stock-outs; too much ties up cash.

Method 1: Max-min (simple)

Safety stock = (Max daily sales × Max lead time) − (Average daily sales × Average lead time)

Input	Value
Max daily sales	30
Max lead time	10 days
Average daily sales	20
Average lead time	7 days
Safety stock	(30 × 10) − (20 × 7) = 160

It is easy but sensitive to a single extreme day.

Method 2: Statistical (service level)

Safety stock = Z × σ(daily demand) × √(Lead time)

Z is the service-level factor: about 1.28 for 90%, 1.65 for 95% and 2.33 for 99%. σ is the standard deviation of daily demand, and lead time is in days.

Input	Value
Service level	95% (Z = 1.65)
Std deviation of daily demand	8 units
Lead time	9 days (√9 = 3)
Safety stock	1.65 × 8 × 3 = about 40 units

If lead time varies too

Add the variability of lead time to the formula, or use the longest realistic lead time. Keep your actual supplier receipts as the source.

Choosing a service level

A 99% service level costs far more stock than 95%. Set higher levels for A-class and critical items and lower for C-class (see ABC analysis).

In NexoraIMS

Service levels, review cycle and default lead time are set under Planning settings on the Forecasts and replenishment page, where the system estimates demand from your own sales history. These are estimates; review them. See reorder point software and how to calculate reorder point.