Safety Stock

Why safety stock matters for an FBA seller

Forecasts are never exact and suppliers are never perfectly on time. Safety stock is what stands between a routine bad week and a lost Buy Box. Without it, every forecast miss or shipping delay becomes a stockout.

The trade-off is real and worth naming. Too little safety stock and you stock out. Too much and you carry extra units that age toward storage fees and the aged inventory surcharge. The goal is the right buffer per SKU, sized to its actual demand and supply volatility, not a blanket number across the catalog.

Measure safety stock two ways: units and days of cover

A unit count alone is not enough to know whether a buffer is safe. 500 units of safety stock is a fortress for a SKU that sells 5 a day and a fire drill for one that sells 100 a day. Inventory Hero tracks safety stock both ways and surfaces both numbers on every SKU.

Units is the operational number you order against: when reorder point math says you need 1,200 units on hand, that 1,200 is a unit count. Days of cover is the planning number you reason with: a 1,200 unit buffer for a SKU selling 40 units per day is 30 days of cover, which is comfortable; the same 1,200 units against 200 units per day is 6 days of cover, which is dangerously thin.

Carrying both views lets you spot the problem the units alone hide. A SKU can be sitting at its target unit buffer while the days-of-cover number is quietly collapsing because velocity climbed; or units can look low while days of cover is still healthy because demand softened. The two numbers together tell you which one is actually moving.

Rule of thumb: aim for at least 30 days of safety stock

A good default starting point for most Amazon FBA sellers is 30 days of safety stock on top of expected demand during your lead time. Thirty days is roughly the cushion that absorbs a normal demand spike, a supplier slipping a couple of weeks, a freight delay, and an FBA check-in queue, without forcing you to airfreight or accept a stockout.

Treat 30 days as the floor, not the answer. Volatile SKUs, hard-to-resupply products, seasonal peaks, and slow lead times all justify pushing higher. Highly predictable, fast-to-resupply SKUs can run thinner. Inventory Hero lets you set the days-of-cover target per SKU and then translates that into the unit buffer you actually order against, so the rule of thumb stays human-readable while the math underneath stays SKU-specific.

How safety stock connects to your restock decisions

Safety stock is a direct component of your reorder point: you reorder when on-hand inventory reaches lead-time demand plus safety stock. Raise the safety stock and the reorder trigger moves earlier; lower it and you order later but with less protection.

Volatile, high-velocity, hard-to-resupply SKUs justify a deeper buffer. Predictable, easily reordered SKUs need less. Sizing it by SKU instead of one global rule is how you stay in stock without drowning in excess inventory.