Safety Stock Calculator
Safety stock is the buffer inventory you hold to absorb demand and lead-time variability so you don't stock out before a replenishment arrives. The statistical method sets it to Z × the standard deviation of daily sales × the square root of lead time. Enter your numbers below to size your buffer instantly.
Add this buffer on top of your lead-time demand to get your reorder point.
The formula
Worked example
Selling 20 units/day with a 10-day lead time, a 95% service level (Z = 1.65) and a demand std. deviation of 6 units: safety stock = 1.65 × 6 × √10 ≈ 32 units. Hold those 32 units on top of your expected lead-time demand to stay 95% protected against stock-outs.
Stop calculating by hand
Logistified computes this across your whole Shopify catalog automatically — from live sales data — and turns it into reorder alerts and purchase orders.
Frequently asked
What is safety stock?
Safety stock is extra inventory held to protect against running out when demand spikes or a supplier is late. It's a buffer on top of the stock you expect to sell during lead time, sized to your target service level — the probability you won't stock out in a given replenishment cycle.
How do I estimate my demand standard deviation?
Ideally, take the standard deviation of your daily unit sales over several recent weeks. If you don't have that, a common rule of thumb is roughly 30% of average daily sales — so 20 units/day implies a σ near 6. The more variable your sales, the larger the buffer you'll need.
Does a higher service level always mean more safety stock?
Yes. The Z-factor grows with the service level (95% ≈ 1.65, 99% ≈ 2.33), so chasing the last few percent of availability costs disproportionately more buffer. Most retailers target 95–98% on important SKUs and accept lower levels on long-tail items.