Safety Stock & Reorder Point Calculator
Protect against demand variability: enter demand, lead time, service level and variability to get safety stock and the reorder point. Free, instant and fully in your browser.
Reorder point
466
units
SS = Z × σ_d × √LT · ROP = demand × LT + SS
The service factor Z comes from the target service level via the normal distribution. Safety stock buffers demand variability over the lead time; reorder point triggers replenishment before stock runs out.
What is safety stock and reorder point?
Safety stock is the buffer inventory that protects against variability in demand during the replenishment lead time, so you do not run out before new stock arrives. It is driven by how variable demand is and how high a service level you want — a higher target means a larger service factor (Z) and more buffer. The reorder point is the stock level that triggers a new order: enough to cover expected demand over the lead time plus the safety stock.
How to calculate safety stock
- 1
Characterise demand
Enter average daily demand and the standard deviation of daily demand, ideally from historical data.
- 2
Set lead time and service level
Enter the replenishment lead time in days and the service level you want to hold (e.g. 95%), which sets the service factor Z.
- 3
Read safety stock and reorder point
Safety stock = Z × demand std. dev × √lead time. Reorder point = average daily demand × lead time + safety stock.
Frequently asked questions
What service level should I choose?
Higher service levels cut stockouts but cost more inventory. 90–98% is common; the right number depends on the cost of a stockout versus the cost of holding stock. The service factor Z grows non-linearly as you approach 100%.
Why is lead time under a square root?
Demand variability accumulates over the lead time, and the standard deviation of independent daily demand scales with the square root of the number of days — so safety stock grows with √lead time, not lead time itself.
How does Yamazo Studio help?
Shorter, more reliable lead times shrink safety stock. Yamazo Studio analyses changeover and process work from video (SMED, standard work), helping cut the internal lead time that inflates buffers.
Related lean & inventory tools
Shrink buffers with shorter lead times
Safety stock falls when lead times shorten and stabilise. Yamazo Studio turns process video into SMED and standard work — one offline Lean Operating Desk.
Get your free demo