Start even if you do not know standard deviations
Safety Stock & Reorder Point Calculator (Demand and Lead Time)
Estimate safety stock and reorder points from demand and lead time variability.
Supports min/max estimation, low/medium/high service levels, and period-based calculations for seasonal demand.
Inputs, share URLs, and on-device storage stay inside this browser and are not sent to a server.
See the assumptions and calculation logic clearly
Review busy and slow seasons by period
Reuse results quickly with copy and share URL
How to use
- Choose the demand average and how you want to enter variability.
- Enter the lead time average and variability, then choose a service level.
- Turn on seasonality if needed and enter multiple periods with separate demand values.
- Copy the result or save it as a shareable URL for the next reorder-point review.
Sample
Standard case
Input
Average demand 120/day, demand 80 to 160, average lead time 7 days, lead time 5 to 10 days, service level 95%
Output
Reorder point 1,103, safety stock 263, average demand × lead time 840
With seasonality
Input
Peak season 180 (140 to 240), normal season 120 (80 to 160), slow season 70 (50 to 95), average lead time 7 days, lead time 5 to 10 days
Output
The peak season is highlighted as the highest reorder-point period, and each period shows its own safety stock and reorder point.
Quick safety stock and reorder point calculation with lead time variability
Review safety stock and reorder point on the same screen.
The tool assumes demand variability and lead time variability are independent and uses a normal approximation.
Higher service levels increase both safety stock and reorder point.
The more lead time varies, the more safety stock tends to matter.
What to enter when variability is unknown
Starting with a min/max estimate is usually good enough.
If you have historical data, enter the average and standard deviation directly.
If you are unsure, start with Medium (95%) as the service level.
The rough low / medium / high option is useful for an initial comparison.
Glossary
Safety stock
Extra inventory kept to absorb demand or lead time variability.
Reorder point
The stock level where you should place the next order.
Service level
Here it means the approximate probability of not stocking out during lead time when you reorder at the reorder point.
Lead time variability
How much ordering-to-receipt time varies in days or weeks.
How the calculation works
- σDL = sqrt((μL × σD²) + (μD² × σL²))
- Safety stock SS = z × σDL
- Reorder point ROP = (μD × μL) + SS
- Min/max estimate: σ ≈ (max - min) ÷ 4 (assuming about 95% coverage)
FAQ
I do not know the standard deviation.
You can estimate it from min and max values. The tool treats that range as roughly 95% of the usual spread and estimates the standard deviation automatically. That is often practical enough to start with.
What does service level mean here?
Here it is the approximate probability of not stocking out during lead time when you reorder at the reorder point. Higher targets usually increase safety stock.
My item has seasonal demand.
You can calculate multiple periods separately. Enter average demand and variability for busy, normal, and slow periods, then compare each period's safety stock and reorder point.
I have the reorder point. Will this tool also tell me order quantity?
No. This tool estimates when to reorder. Order quantity still depends on MOQ, lot size, storage capacity, cost, and ordering policy.
Is this the same as fill rate?
No. The service level in this tool is an approximation of cycle service level. It does not calculate fill rate directly.
Notice
- These results are only a planning guide. You still need to consider MOQ, order multiples, storage space, budget, and obsolescence risk.
- The tool assumes demand variability and lead time variability are independent and uses a normal approximation.
- For intermittent demand or frequent supply disruptions, a different model may fit your operation better.