Free tool
Safety stock calculator
Work out how much buffer stock to hold so a slow supplier or a busy week doesn't leave you sold out. Two methods: simple days of cover, or the statistical formula.
Standard deviation: how much daily sales swing around the average. A spreadsheet's STDEV of your last 28 days of daily unit sales gives it.
Method 1: days of cover
Easy to explain and good enough for most stores. Sell 6 a day and want a week of buffer: 6 × 7 = 42 units.
Method 2: statistical safety stock
Z comes from the service level you want (the chance you don't run out during a lead time): 1.28 for 90%, 1.65 for 95%, 1.96 for 97.5%, 2.33 for 99%. With σ = 2.5 and a 21-day lead time at 95%: 1.65 × 2.5 × 4.58 = 19 units.
The statistical method rewards steady sellers with a smaller buffer and gives volatile ones a bigger one. It assumes your lead time is reliable. If it isn't, use the simple method with more days.
From safety stock to reorder point
Plug both into the reorder point calculator to get an order-by date and quantity, or read the forecasting guide.
FAQ
What is a good safety stock level?
For most small Shopify stores, 7 to 14 days of average sales is a sensible starting point. Use more for long or unreliable lead times and for best-sellers where a stockout is expensive.
What is the safety stock formula?
Simple: average daily sales x safety days. Statistical: Z x standard deviation of daily sales x square root of lead time in days, where Z sets your service level (1.65 for 95%).
Is safety stock the same as reorder point?
No. Safety stock is the buffer. The reorder point is lead-time demand plus that buffer: daily sales x lead time + safety stock.
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