Safety stock formula: how much extra inventory should you keep?

Updated October 28, 2026 · SellerSheetKit Guides

Safety stock is the extra inventory you keep on hand so a slow supplier or a surprise busy week doesn't leave your best seller out of stock. Too little and you lose sales (and on some marketplaces, ranking). Too much and cash sits on a shelf. The goal is a number you can defend, not a gut feeling.

The simple formula (good for most small shops)

Safety stock = (max daily sales × max lead time) − (average daily sales × average lead time)

It compares a bad-case month with a normal month. Whatever the gap is, that's the cushion you need to ride out the bad case.

Worked example (example numbers)

Input	Value
Average daily sales	4 units
Busiest-day sales (typical peak, not a one-off viral day)	7 units
Average supplier lead time	10 days
Longest recent lead time	14 days
Safety stock	(7 × 14) − (4 × 10) = 98 − 40 = 58 units

That can feel like a lot. If it ties up too much cash, use realistic peaks (your 90th-percentile day over the last few months) rather than the single biggest day you've ever had.

The demand-variability formula (for steadier, higher-volume SKUs)

Safety stock = Z × standard deviation of daily sales × √(lead time in days)

Z is a service-level factor: about 1.28 for roughly 90% of cycles without a stockout, 1.65 for about 95%, and 2.33 for about 99%. You can get the standard deviation of daily sales with =STDEV(range) in Google Sheets.

Example numbers: daily sales vary with a standard deviation of 1.5 units, lead time is 9 days, and you want about 95% coverage. Safety stock = 1.65 × 1.5 × √9 = 1.65 × 1.5 × 3 ≈ 7.4, so round up to 8 units. This version assumes lead time is fairly stable; if your supplier is unpredictable, the simple formula above is usually the safer pick.

How safety stock fits your reorder point

Reorder point = (average daily sales × average lead time) + safety stock

Using the first example: (4 × 10) + 58 = 98 units. When stock on hand drops to 98, place the next order. Our reorder point guide walks through this step by step.

Which products need it most

Best sellers, where a stockout costs the most sales and reviews.
Items with long or unreliable supplier lead times, including imports (see landed cost).
Seasonal items heading into their peak, where you should recalculate with recent peak sales.
Components and packaging you can't sell without, even if the finished product is made to order.

Slow movers and made-to-order items often need little or none. Checking your inventory turnover helps you spot where extra stock is just tying up cash.

Common mistakes

Setting it once and never revisiting it. Recalculate when sales or lead times change, at least every quarter.
Using one number for every SKU. Fast and slow sellers need very different cushions.
Forgetting stock that's already on order when deciding whether to reorder.
Ignoring shipping and receiving time on your side, which is part of real lead time.

Educational content only, not financial or business advice. Example numbers are illustrative; use your own sales history.

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