Safety Stock Calculator

Safety stock is the extra inventory kept on hand to guard against stockouts caused by demand variability and supply lead time uncertainty. It acts as a buffer between your reorder point and the minimum stock level needed to avoid running out of product.

The standard formula uses a Z-score (based on your desired service level) multiplied by the square root of (lead time variance x demand variance). The specific formula is: Safety Stock = Z x sqrt((avg lead time x demand std dev^2) + (avg demand^2 x lead time std dev^2)).

A 95% service level means you will be in stock 95% of the time. For critical or high-margin products, use 97.5% or 99%. For low-cost, easily substituted items, 90% may be sufficient. Higher service levels require exponentially more safety stock.

The reorder point (ROP) is the inventory level at which you should place a new order. It equals (average daily demand x average lead time) + safety stock. When your inventory drops to the ROP, a new order ensures stock arrives before you run out.

Higher lead time variability means more uncertainty in when your supplier will deliver, requiring more safety stock. If your supplier always delivers in exactly 7 days (std dev = 0), you need less buffer than if delivery ranges from 5 to 12 days.