Safety Stock Calculator
Size safety stock from demand variability, lead time and the service level you want to hold.
About Safety Stock Calculator
Safety stock scales with the square root of lead time, not lead time itself, because variability partially cancels over a longer window. Raising service level from 95% to 99% increases the Z value by about 40%, which is why the last few points of availability are disproportionately expensive. Halving supplier lead time cuts required safety stock by about 30%, which is usually cheaper than financing the extra inventory.
Formula
Safety Stock
Safety stock = Z × standard deviation of demand × √lead time
- Z
- Standard normal value for the target service level
- lead time
- Days between placing and receiving an order
Worked example
With the default values below, Safety Stock Calculator returns Safety Stock Required of 185. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Average daily demandminimum 0 units
- 120 units
- Standard deviation of daily demandminimum 0 units
- 30 units
- Supplier lead timeminimum 0 days
- 14 days
- Target service levelaccepts 50–99.9 %
- 95 %
Results
- Safety Stock Required
- 185 units
- Service Level Z-Value
- 1.645
- Expected Demand During Lead Time
- 1,680 units
- Reorder Point
- 1,865 units
- Days of Cover the Buffer Provides
- 1.5 days
Step by step
- 1Values used
- 2Safety Stock
- 3Safety Stock Required= 185 units
- 4Service Level Z-Value= 1.645
- 5Expected Demand During Lead Time= 1,680 units
- 6Reorder Point= 1,865 units
- 7Days of Cover the Buffer Provides= 1.5 days
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
- This is an estimate for general information and not professional financial, tax or legal advice. See the full disclaimer for the limits that apply to this kind of calculator.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Safety Stock calculated?
Safety stock = Z × standard deviation of demand × √lead time Safety stock scales with the square root of lead time, not lead time itself, because variability partially cancels over a longer window. Raising service level from 95% to 99% increases the Z value by about 40%, which is why the last few points of availability are disproportionately expensive.
Why does Safety Stock matter?
Halving supplier lead time cuts required safety stock by about 30%, which is usually cheaper than financing the extra inventory.
What values do I need to enter?
This calculator takes 4 inputs: Average daily demand, Standard deviation of daily demand, Supplier lead time, Target service level. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.