Rex says
The fast lane for the math you almost remember from school. Type the numbers, get the answer, move on with your day.
Try a scenario
Click to load — tweak from there.Inputs
Result
Safety stock
627
Implied reorder point
2,067
Average demand during lead time
1,440
Z-score used
1.65
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How to use this
- 1Enter average daily demand (units).
- 2Enter std deviation of daily demand (units).
- 3Enter average lead time (days).
- 4Enter std deviation of lead time (days).
- 5Enter target service level.
- 6Read your safety stock on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Safety stock exists to absorb the combined variability of demand and supplier lead time so you don't stock out between reorders. This calculator uses the standard statistical formula that combines demand standard deviation, lead time standard deviation, and a z-score representing your target service level (probability of not stocking out in a cycle). A 95% service level uses z=1.65, 97.5% uses z=1.96, and 99% uses z=2.33 — moving from 95% to 99% roughly doubles required safety stock for most SKUs because the tail of the normal distribution gets expensive fast. This is why blanket 99% service level targets across an entire catalog are usually a mistake; reserve high z-scores for A-items and critical SKUs, and accept lower service levels on C-items where a stockout is cheap.
Worked example
Using the values the calculator loads with:
Inputs
- Average daily demand: 120 units
- Std deviation of daily demand: 35 units
- Average lead time: 12 days
- Std deviation of lead time: 3 days
- Target service level: 95%
Results
- Safety stock: 627
- Implied reorder point: 2,067
- Average demand during lead time: 1,440
- Z-score used: 1.65
What each field means
Inputs
- Average daily demand (units)
- The average daily demand used in the calculation, measured in units. Starts at 120 units so you have a working example on load.
- Std deviation of daily demand (units)
- The std deviation of daily demand used in the calculation, measured in units. Starts at 35 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 12 days so you have a working example on load.
- Std deviation of lead time (days)
- The std deviation of lead time used in the calculation, measured in days. Starts at 3 days so you have a working example on load.
- Target service level
- Pick the option that matches your situation — the maths changes per option. Choices: 90%, 95%, 97.5%, 99%, 99.5%.
Results
- Safety stock
- 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.
- Implied reorder point
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Average demand during lead time
- 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
Where do I get demand and lead time standard deviation?
Pull 6-12 months of daily order history for the SKU and compute the standard deviation in a spreadsheet with STDEV. For lead time, use actual receipt dates versus PO dates from your last 10-20 purchase orders per supplier, not the supplier's quoted lead time, which is almost always optimistic.
Why does lead time variability matter as much as demand variability?
The formula weights both because a supplier that's sometimes 8 days and sometimes 20 days late creates stockout risk even if your demand is perfectly steady. In practice, unreliable suppliers with high lead time variance often need more safety stock than volatile-demand items with a rock-solid supplier.
Should every SKU have the same service level?
No. Segment by ABC/XYZ — A-items with high margin or stockout cost justify 98-99% service levels, while C-items with cheap substitutes or low margin can run 90-92% and free up working capital for where it matters more. A flat 99% policy across the catalog usually ties up cash in safety stock you don't need on slow movers.
How does this change with a shorter lead time?
Safety stock scales with the square root of lead time, so cutting lead time in half doesn't halve safety stock — it cuts it by about 30%. Still meaningful, and it's why negotiating faster replenishment or dual-sourcing a critical component often beats carrying more inventory as a hedge.
Accuracy and limitations
- Results are rounded for display; the underlying calculation keeps full precision.
- Very large or very small inputs may hit floating-point limits in the browser.
- Inputs outside the accepted range are clamped rather than rejected.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Safety Stock Calculator (Service Level). Retrieved from https://www.revenuelab.fyi/toolbox/safety-stock-service-level
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/safety-stock-service-level" target="_blank" rel="noopener">Safety Stock Calculator (Service Level) — RevenueLab</a> (2026).</p>
Source: [Safety Stock Calculator (Service Level) — RevenueLab](https://www.revenuelab.fyi/toolbox/safety-stock-service-level) (2026).