Safety Stock Days Calculator
Buffer stock from demand and lead time variability.
Enter your values
1.65 = 95% service level
Enter the daily demand std deviation in units.
Enter the lead time in days.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Safety stock
- 42 units
- Days of extra cover
- 2.6
- Service level factor used
- 1.65
- Lead time
- 10 days
Quick answer
Safety stock scales with demand variability, lead time and the desired service level, using the classic square-root formula.
SS = Z x std dev of demand x sqrt(lead time)
At a glance
| What it does | Buffer stock from demand and lead time variability. |
|---|---|
| Category | Business |
| Inputs needed | Service level factor (Z), Daily demand std deviation, Lead time |
| Main output | Safety stock |
| Formula | SS = Z x std dev of demand x sqrt(lead time) |
| Cost | Free — no sign-up, no download |
How to use the Safety Stock Days Calculator
- 1Enter your target service level as a Z-score (1.65 for 95%, 2.33 for 99%).
- 2Enter the standard deviation of daily demand.
- 3Enter supplier lead time in days.
Inputs explained
- Service level factor (Z)
- 1.65 = 95% service level
- Daily demand std deviation(units)
- Enter the daily demand std deviation in units.
- Lead time(days)
- Enter the lead time in days.
Worked example
Using the values the calculator loads with:
Inputs
- Service level factor (Z)1.65
- Daily demand std deviation8 units
- Lead time10 days
Results
- Safety stock42 units
- Days of extra cover2.6
- Service level factor used1.65
- Lead time10 days
Frequently asked questions
Where do I get demand std deviation?
Calculate it from your last several months of daily sales data.
Higher Z-score means what?
A higher Z-score reduces stockout risk but ties up more cash in inventory.
What do I need to enter into the Safety Stock Days Calculator?
Just 3 values: service level factor (z), daily demand std deviation, lead time. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Safety Stock Days Calculator work out the answer?
Safety stock scales with demand variability, lead time and the desired service level, using the classic square-root formula. It applies the formula SS = Z x std dev of demand x sqrt(lead time) and shows the working so you can check each step by hand.
Is the Safety Stock Days Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.