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 to 98% on important SKUs and accept lower levels on long-tail items.