What is safety stock, and how is it calculated?
Safety stock is inventory held to protect against variability in demand and supply during the replenishment lead time.
Why companies hold safety stock
If demand and lead times were perfectly predictable, stock could arrive exactly when the last unit was used. In practice, usage varies and suppliers are sometimes late. Safety stock is the buffer that covers that uncertainty, so that a part is still available while a replenishment order is on its way.
The basic formula
When demand varies but lead time is reliable:
When both demand and lead time vary:
| Symbol | Meaning |
|---|---|
|
Z |
Service level factor (see next table) |
|
d |
Average demand per period |
|
σd |
Standard deviation of demand per period |
|
L |
Average lead time, in the same periods |
|
σL |
Standard deviation of lead time |
Service level and Z
The service level is the probability of not running out during a replenishment cycle. Higher service levels need disproportionately more stock.
| Service level | Z |
|---|---|
|
90% |
1.28 |
|
95% |
1.65 |
|
98% |
2.05 |
|
99% |
2.33 |
|
99.9% |
3.09 |
| Worked example | |
|---|---|
|
Average demand |
20 units per week |
|
Demand standard deviation |
8 units per week |
|
Average lead time |
4 weeks |
|
Lead time standard deviation |
1 week |
|
Service level |
95% (Z = 1.65) |
Safety stock = 1.65 × √(4 × 8² + 20² × 1²) = 1.65 × √656 ≈ 43 units.
Reorder point = average demand during lead time + safety stock = 80 + 43 = 123 units.
If lead time were perfectly reliable, safety stock would be only 27 units. Supplier variability adds 16 units of buffer, which is why lead-time data matters as much as demand data.
Why standard formulas fail for slow movers
The formulas assume demand follows a normal distribution. For spares that move a few times a year, it does not. Demand is mostly zero, with occasional spikes, and the formula can recommend far too little. Slow-moving and critical items need a different approach: first decide whether to stock, then use methods built for intermittent demand.
How StoXeo calculates safety stock
StoXeo applies different policy methods per demand profile, and shows the inputs and results of every calculation.
Frequently asked questions
What is the difference between safety stock and a reorder point?
Safety stock is the buffer. The reorder point is the stock level that triggers a new order: expected demand during the lead time plus safety stock.
Should every item have safety stock?
No. Items with predictable, planned demand may need little or no safety stock, and some slow movers are better managed with a stock-or-no-stock decision than with a safety stock formula.