Rex says
Money math without the spreadsheet headache. Plug in your numbers and I'll show you exactly where the dollars land.
Try a scenario
Click to load — tweak from there.Inputs
Result
Safety stock (units)
59
Safety stock at 99% service level
83
Z-score used
1.65
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How to use this
- 1Enter daily sales standard deviation (units).
- 2Enter average lead time (days).
- 3Enter desired service level.
- 4Read your safety stock (units) on the right — it updates as you type.
- 5Hit Share to keep the scenario or send it to someone.
About this calculator
Safety stock exists to absorb the two things that cause stockouts even when your reorder point math is correct on average: demand spikes above your typical sales velocity, and supplier lead times running longer than planned. Sizing it off gut feel usually means either chronic stockouts or an oversized buffer quietly eating cash flow. This calculator uses the standard formula that combines demand variability (standard deviation of daily sales) and lead time in days, scaled by a service-level Z-score, to produce a safety stock unit count sized to your desired in-stock probability. Enter your daily sales standard deviation (how much daily sales typically swings above or below average), average lead time in days, and desired service level as a percentage (95% and 99% are the common tiers). Higher service levels require disproportionately more safety stock as you approach 99%+, so this calculator also shows the tradeoff by service level so you can pick a level appropriate to the SKU's margin and stockout cost.
Worked example
Using the values the calculator loads with:
Inputs
- Daily sales standard deviation: 6 units
- Average lead time: 35 days
- Desired service level: 95% (Z=1.65)
Results
- Safety stock (units): 59
- Safety stock at 99% service level: 83
- Z-score used: 1.65
What each field means
Inputs
- Daily sales standard deviation (units)
- The daily sales standard deviation used in the calculation, measured in units. Starts at 6 units so you have a working example on load.
- Average lead time (days)
- The average lead time used in the calculation, measured in days. Starts at 35 days so you have a working example on load.
- Desired service level
- Pick the option that matches your situation — the maths changes per option. Choices: 90% (Z=1.28), 95% (Z=1.65), 98% (Z=2.05), 99% (Z=2.33).
Results
- Safety stock (units)
- Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Safety stock at 99% service level
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Z-score used
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How do I calculate my daily sales standard deviation?
Pull at least 60-90 days of daily unit sales, calculate the average, then compute the standard deviation in a spreadsheet with the STDEV function. If you don't have granular daily data yet, a rough shortcut is 30-40% of your average daily sales as a starting estimate.
Why does going from 95% to 99% service level cost so much more inventory?
The Z-score doesn't scale linearly with service level — it grows faster as you approach 100% because you're covering increasingly rare, extreme demand spikes. Going from 95% to 99% typically raises required safety stock by 40%, while 99% to 99.9% can nearly double it again.
Should every SKU use the same service level?
No. Reserve 98-99% service levels for high-margin bestsellers or products where a stockout damages a customer relationship (like a subscription core item), and use 90-95% for lower-margin or slow-moving SKUs where a stockout is a minor inconvenience, not a lost customer.
Does this formula account for lead time variability too?
This version assumes lead time itself is roughly fixed and only demand varies. If your supplier's lead time also swings significantly, use an extended formula that adds a term for lead time variability, since that risk compounds with demand risk rather than replacing it.
Accuracy and limitations
- Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
- Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.
- This is educational maths, not financial advice. Check anything contractual with the lender or your accountant.
Related tools
Inventory Reorder Point Calculator
The inventory level that triggers a new purchase order before you stock out.
Economic Order Quantity (EOQ) Calculator
The order size that minimizes total ordering and holding cost.
Sell-Through Rate Calculator
What percentage of received inventory has actually sold in a period.
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Safety Stock Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/safety-stock-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/safety-stock-calculator" target="_blank" rel="noopener">Safety Stock Calculator — RevenueLab</a> (2026).</p>
Source: [Safety Stock Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/safety-stock-calculator) (2026).