How Much Safety Stock Should You Actually Hold?
Safety stock is the buffer inventory you keep on hand to survive supplier delays or a sudden spike in demand without running out. Hold too much and you tie up cash in storage costs; hold too little and a stockout costs you sales you can't easily get back. The right amount depends on three things: your target service level, how much your demand varies day to day (its standard deviation), and your supplier's lead time. The longer the lead time, the more your demand varies, and the higher your target service level, the more safety stock you need. This calculator takes those three inputs and instantly gives you the recommended quantity along with the annual and monthly cost of holding it.
Safety stock isn't a number you set once and forget — it needs to be recalculated whenever a supplier changes, or when demand variability shifts between your busy and slow seasons. Piling excess safety stock onto slow-moving SKUs adds not just holding cost but obsolescence risk, so it's worth applying different service level targets to different items rather than using one number across your whole catalog.
Frequently Asked Questions
It's the probability of not running out of stock before your next shipment arrives. A 95% service level means you'll avoid a stockout in roughly 95 out of every 100 lead-time cycles. Raising the target increases the safety stock you need.
Yes. This formula scales safety stock with the square root of lead time, so items sourced overseas with long lead times need proportionally more buffer than items you can reorder quickly.
Calculate the standard deviation of your actual daily or weekly sales over the last few months. If you don't have enough data yet, using 20-30% of average demand as a rough placeholder is a reasonable start.
※ Real demand distributions may differ from the normal distribution assumed here. This is an estimate for reference only.