Safety stock is buffer inventory held above your normal replenishment quantity. It exists to cover two risks: forecast error (actual demand exceeds what was predicted) and lead time variability (a supplier shipment arrives later than expected).

P21 only applies safety stock calculations when using the dynamic replenishment methods — EOQ (Economic Order Quantity) and Up To. If an item uses Min/Max or OPOQ, safety stock must be manually built into the minimum level or order point instead.

When PORG (Purchase Order Requirements Generation) runs, P21 adds the calculated safety stock quantity on top of the standard recommended purchase quantity for each eligible item.

You can define safety stock parameters at four levels. When generating purchasing requirements, P21 checks each level top-to-bottom and stops at the first one that has a value set:

1. 1Item / Location — Most specific — overrides everything below
2. 2ABC Class (Purchase Class) — Applies to all items in the class
3. 3Supplier — Applies to all items from this supplier
4. 4Run Criteria — Fallback used in Safety Stock Analysis

The hierarchy controls which parameters are used, not the final quantity. P21 still calculates safety stock per item — it just pulls the settings from whichever level wins.

The simplest method. You manually enter a number of days of safety stock. P21 multiplies that day count by the item's forecasted daily usage to arrive at a quantity.

Example: 7 days of safety stock on an item with 10 units/day usage = 70 units added to the purchase recommendation.

P21 looks back a set number of demand periods and compares forecasted usage against actual usage. It collects all periods where actual exceeded forecast, sorts the variances, and takes the median average deviation.

That median is multiplied by a configurable Deviation Multiplier (separate values for 1, 2, or 3+ shortfall periods). The result is converted to days of stock, then clamped between the configured Min and Max Safety Stock Days.

Critical items use a separate, higher multiplier and separate Min/Max bounds — flagged per item in Item Maintenance → Replenishment tab.

Instead of looking at historical deviation, this method works backwards from a target fill rate. You set a Service Level % Goal — say 98% — and P21 calculates how much safety stock is needed to achieve that fill rate given the item's demand variability.

The system uses the item's Mean Absolute Percent Error (MAPE) and standard deviation of demand to determine the required buffer. Higher service level goals require more safety stock; items with erratic demand require more than items with steady demand.