Safety Stock Formula
How apparel allocators calculate safety stock — the buffer that protects against demand volatility and lead-time variability.
What Safety Stock measures
Safety stock is the inventory buffer held above expected demand to protect against demand spikes and lead-time variability. It is what keeps a service-level target intact when the average demand forecast misses.
Safety Stock = Average Weekly Demand × Safety-Days Coverage ÷ 7More rigorous safety-stock formulas use z-scores (service-level targets) and standard deviation of demand. The simple days-coverage formulation above is the practical apparel version most planners actually use.
Worked apparel example
A core tee selling 40 units/week, with 7 safety-days of coverage (standard for mid-market DTC).
Safety Stock = 40 × 7 ÷ 7 = 40 units
40 units of buffer above the replenishment trigger. For a volatile style — fashion piece with high variability — 14 days of safety stock is more typical. For a core basic with predictable velocity, 5–7 days.
Hold 40 units as safety buffer. Higher for volatile demand or unreliable lead times; lower for core basics with predictable velocity.
How to set safety-days by style type
| Style type | Typical safety days | |---|---| | Core basic, stable demand | 5–7 days | | Fashion style, moderate variability | 10–14 days | | New launch, no history | 14–21 days, then decay | | Long lead-time import | 14–21 days | | Short lead-time domestic | 3–5 days |
Failure modes we see
One safety-stock rule across all styles. Fast-moving core carries the same safety days as volatile fashion. Core over-stocks; fashion stocks out. One rule cannot serve both demand profiles.
How RetailNorthstar handles safety stock
Safety-days rules attach to style type, not style-by-style. When style demand stabilizes (moves from fashion to core), the rule updates. Stock-outs and over-stocks attributable to safety-stock miscalibration are flagged in hindsight.
Related formulas
- Replenishment Trigger — safety stock is a trigger component
- Weeks of Supply — the velocity input
See safety-stock rules by style type — managed as policy, not spreadsheet cell.
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