Buffer inventory to prevent stockouts at your target service level.
Inputs
Daily demand variability
For working-capital calc
Results
Safety stock—units
Reorder point
—
Days of cover
—
Capital tied up
—
Service-level trade-offs
Service Level
Safety Stock
Reorder Point
Capital
About this calculator
Safety stock is the inventory operators love to hate. Hold too little and you stock out, lose sales, and damage brand. Hold too much and your working capital is locked in product instead of growth. The optimal level depends on three things: how variable your demand is, how variable your lead times are, and how high a service level you want to maintain.
The standard formula — Safety Stock = Z × σ × √(Lead Time) — converts your demand variability and target service level into a buffer in units. The Z-multiplier comes from the normal distribution: 95% service level means stockout 5% of cycles, which corresponds to Z=1.65 standard deviations above the mean. Higher service level = bigger Z = more safety stock.
The reorder point is safety stock + average lead-time demand. When inventory hits the reorder point, place the next purchase order. The math assumes your supplier delivers on time — if lead times themselves are variable (most are), you need more safety stock to cover supplier-side delays in addition to demand spikes.
Track stockout rate as a check on the math. If you\'re carrying 95%-target safety stock but stocking out 15% of the time, your demand variability is higher than your standard deviation calculation captured. Could be seasonality, a recent campaign spike, or a shift in product mix. Pair this with the Reorder Point Calculator and the Inventory Turnover Rate tool for fuller inventory planning.
Frequently asked questions
What is safety stock?
Safety stock is the buffer inventory you hold to absorb demand spikes and supplier delays. It sits on top of your average lead-time demand. If you sell 50 units/day with a 10-day lead time, average lead-time demand is 500 — safety stock is whatever extra you carry to handle days when sales spike or shipments are late.
How is safety stock calculated?
Safety Stock = Z × σLT × √(Lead Time). Z is the service-level multiplier (1.65 for 95% service level, 1.96 for 97.5%, 2.33 for 99%), σ is the standard deviation of daily demand, and √Lead Time accounts for cumulative variance over the lead-time window.
What service level should I target?
Most ecommerce: 95% service level (stockout 5% of replenishment cycles). Premium/loyalty-driven brands: 97-99% (stockouts hurt brand more than holding cost). Commoditized/low-margin: 90-93% (holding cost matters more than occasional stockouts). Higher service level = more safety stock = more cash tied up.
What's the cost of running too little safety stock?
Stockouts. Lost sales (customer goes elsewhere) plus brand damage (customer remembers the failure) plus rush-shipping fees if you scramble to expedite. For top SKUs, a stockout often costs 3-5× the holding cost of carrying enough safety stock.
What's the cost of running too much?
Working capital tied up + warehouse storage cost + obsolescence risk + opportunity cost of cash. For DTC brands, every 30 days of extra inventory typically ties up 8-10% of annual revenue in working capital.