Free FBA tool
Safety Stock Calculator
Size the buffer that keeps you in stock through normal demand swings. Enter your sales velocity, how much it varies, your target service level, and real FBA lead times, and get your safety stock with the statistical formula, live. No login, no ASIN, nothing stored.
Your numbers
Not sure on the std dev? Most SKUs sit near 6 to 12 units here (15 to 30% of daily sales) — steady demand is lower, spiky or seasonal is higher.
Lead time (the FBA three-part chain)
Safety stock
130 units
Hold this buffer on top of expected lead-time demand to stay in stock about 95% of the time through normal demand swings.
Days of cover
3.3 days
Buffer in days of demand
Z-score
1.64
95% service level
Reorder point
2,610 units
Lead-time demand + this buffer
Total lead time
62 days
Production + freight + receiving
Next step: take your reorder point of 2,610 units to the restock calculator to size the order.
Each step up in in-stock target costs progressively more buffer. Your selection is highlighted.
How it adds up to a reorder point
- Lead time demand(40 units/day x 62 days)2,480 units
- Safety stock(z 1.64 x 10 sigma x sqrt(62))130 units
- Reorder point(Where this safety stock puts your reorder trigger)2,610 units
Inventory Hero
Let Inventory Hero manage your safety stock
This calculator sizes one SKU at one moment. Inventory Hero tracks every SKU's real demand variability and lead time, sets the safety stock and reorder point for each, and updates them as your sales and suppliers shift.
- Per-SKU demand variability measured from your real sales
- Reorder points that already include the right buffer
- Alerts before you dip into safety stock, not after
No credit card required.
The safety stock formula
Statistical safety stock
Safety stock = z (service level) x demand std dev x square root of lead time
Lead time = supplier production days + freight transit days + Amazon receive and check-in days. Because the formula uses the square root of lead time, a longer pipeline raises the buffer, but less than proportionally.
The z-score converts your target in-stock rate into standard deviations: 1.64 for 95%, 2.05 for 98%, 2.33 for 99%. Higher service levels need disproportionately more buffer, which is the tradeoff the comparison chart makes visible.
Worked example
A SKU sells 40 units a day. On a normal day, sales swing about 10 units either side of that average, so the demand standard deviation is 10. Lead time is 20 days production, 35 days freight, and 7 days Amazon receiving, for 62 days total. You want a 95% service level, which is a z-score of about 1.64.
- Safety stock: 1.64 x 10 x square root of 62 (about 7.9) = about 130 units
- Lead-time demand: 40 x 62 = 2,480 units
- Reorder point: 2,480 + 130 = 2,610 units
Hold 130 units of buffer, and place your next order when sellable stock hits 2,610. Take that reorder point straight to the FBA restock calculator to size the order, or read the safety stock guide for the full walkthrough.
Key terms
- Service level
- The probability you stay in stock during a replenishment cycle. A 95% service level means you expect to cover demand without a stockout in 95 of 100 cycles.
- Demand standard deviation
- How much your daily sales vary around the average. The bigger the swings, the more buffer the formula calls for.
- Z-score
- The number of standard deviations that corresponds to your service level. It is the lever that turns a target in-stock rate into a concrete buffer.
Frequently asked questions
What is the safety stock formula?
The statistical safety stock formula is z x demand standard deviation x the square root of lead time. The z-score comes from your target service level (1.64 for 95%, 2.05 for 98%, 2.33 for 99%). Demand standard deviation is how much your daily sales swing, and lead time is the full FBA pipeline: production plus freight plus Amazon receiving.
How do I pick a service level?
Service level is the share of demand cycles you want to cover without stocking out. 95% to 98% is a common range for Amazon. Going higher protects rank but the buffer (and the cash and storage it ties up) climbs fast, because safety stock scales with the z-score. The comparison chart on this page shows that curve for your numbers.
How is safety stock different from a reorder point?
Safety stock is the buffer you hold for variability. The reorder point is the trigger level that includes it: reorder point equals expected lead-time demand plus safety stock. This calculator shows both, so you can take the reorder point straight to the restock calculator.
What demand standard deviation should I use?
Estimate it from how much your daily unit sales bounce around on a normal week. If most days land within plus or minus 10 units of your average, 10 is a reasonable sigma. Steady sellers have a low sigma and need little buffer; spiky sellers have a high sigma and need more.
Does this safety stock calculator account for lead-time variability?
This version models demand variability only, which is the most common form of the formula. It does not yet add lead-time variability. If your Amazon receiving time swings a lot, raise your service level or lead time input to compensate, since unreliable lead times are a real source of stockout risk.
Embed this calculator
Free to use on your own site or blog. Paste this snippet where you want the calculator to appear. It keeps a small credit link back to Inventory Hero.
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<p style="font:13px/1.5 system-ui,sans-serif;text-align:center;margin:8px 0 0"><a href="https://www.inventoryhero.ai/tools/safety-stock-calculator">Safety Stock Calculator</a> by Inventory Hero</p>Inventory Hero
Stop guessing your buffer
Inventory Hero measures each SKU's real demand variability and lead time, then sizes safety stock and reorder points for you, and updates them as your sales and suppliers change.
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