Calculators
Safety Stock Calculator
Calculate safety stock from demand variability, lead time and target service level, with lead time variability included.
- Runs in your browser
- Nothing uploaded
The tool
Enter your numbers below. Results update as you type, and nothing leaves your browser.
Everything you enter stays on your own machine. Nothing is sent to us or to anyone else, which you can confirm in your browser network tab.
What people use it for
- Setting inventory buffers
- Reorder point calculations
- Supplier comparisons
- Inventory policy reviews
About this tool
What the Safety Stock Calculator does
Safety stock protects against running out when demand or supplier lead times vary. The standard formula multiplies a service-level factor (Z) by the standard deviation of demand during the lead time. Including lead time variability gives a more robust buffer.
Formula
- σ (lead-time demand) = √(lead time × σ_demand² + demand² × σ_lead time²)
- Safety stock = Z × σ
- Z comes from the normal distribution for the service level
Worked example
Using the starting values in the calculator above:
| Input | Value |
|---|---|
| Average daily demand | 120 units |
| Standard deviation of daily demand | 30 units |
| Average lead time | 14 days |
| Standard deviation of lead time | 3 days |
| Target service level | 95% |
| Result | Value |
|---|---|
| Safety stock | 621 |
| Days of cover | 5.2 |
| Service level factor (Z) | 1.64 |
| If lead time never varied | 185 |
How to read the result
Going from 95% to 99% service roughly increases safety stock by 40%; reserve high service levels for critical items.
Reducing lead time variability, for example through reliable suppliers, often cuts safety stock more than forecasting does.
Related calculators
- Reorder Point Calculator: Calculate the reorder point from daily demand, lead time and safety stock, and the days until you need to reorder.
- Inventory Turnover Calculator: Calculate inventory turnover, days inventory outstanding and the cash released by improving turns.
- Supplier Lead Time Cost Calculator: Calculate the inventory and carrying cost of a supplier’s lead time and compare a cheaper slower supplier with a faster one.
- Material Lead Time Calculator: Work back from the date materials are needed to the latest order date, adding supplier, shipping, inspection and buffer days.
Browse every calculator in Logistics and Inventory Calculators, or talk to us about websites and portals for logistics companies.
More tools and services
- 1031 Exchange Calculator
- 401(k) Contribution Optimizer
- 529 College Savings Calculator
- A/B Test Significance Calculator
- Accessible Ramp Calculator
- Accounts Payable Aging Calculator
If you would rather someone did this properly for you, we offer Web Design Services and Website Development, SEO Services: Search Engine Optimization, AI Automation Agency.
Questions
Questions about this tool
The things clients actually want to know — costs, timelines, ownership and what happens after launch.
Extra inventory held to protect against variability in demand and supply.
Often 95% for most items, higher for critical or high-margin items.
The number of standard deviations covered for the chosen service level; 1.65 for 95%.
Free strategy session
Let’s build something worth finding.
Tell us where you want the business to be in twelve months. We will come back with a costed plan, a realistic timeline, and the numbers we expect to move — no obligation, no jargon.
- 30-minute call, no pitch deck
- Fixed-price proposal in 48 hours
- You own everything we build
Talk to a developer
Not a sales team
- Response time
- Within 1 business hour
- Where we work
- UK, USA & the Gulf
- Contracts
- No lock-in, monthly rolling
- Working since
- 2023