Inventory planning · Step 1 of the chain
How much safety stock should I hold?
Safety stock is the cushion you keep to cover the gap between what you expect to sell during a replenishment and what actually happens. Put in your numbers and read your buffer below.
Your result
Safety stock to hold for this item
354 units
At a 95% service level with the demand swing you entered and a lead time of 7 days (1 week), hold about 354 units beyond your average lead-time demand of 700 units. That carries you through most replenishment cycles without running out.
Recommendation: lead-time variability is driving most of this buffer. Before adding stock, push your supplier for a tighter, more reliable lead time; shrinking lead-time swing cuts safety stock faster than improving demand forecasts.
Your numbers
Your buffer, drawn to scale
The darker bar is the stock you expect to use while waiting for resupply; the lighter bar is the buffer that absorbs demand and lead-time surprises. Each shows its own unit count, and the dashed line marks your reorder point: the total on-hand level that should trigger a new order, covered by the next tool in the chain.
Where this number comes from, and where it sits in your planning system
The formula
This calculator uses the standard statistical safety-stock formula, which accounts for variability in both demand and lead time:
where Z is the service-level factor, LT is average lead time, σd is demand variability, d is average demand, and σLT is lead-time variability. If your lead time is perfectly steady, the second term falls away and it reduces to the familiar Z × σd × √LT.
Service level and the Z-factor
The service level you pick sets the Z-factor: how many standard deviations of cover you carry. These are the standard values:
| Service level | Z-factor | Plain meaning |
|---|---|---|
| 90% | 1.28 | Out of stock in about 1 cycle in 10 |
| 95% | 1.645 | About 1 cycle in 20 |
| 97.5% | 1.96 | About 1 cycle in 40 |
| 99% | 2.33 | About 1 cycle in 100 |
| 99.9% | 3.09 | Near-continuous cover |
Higher cover is not automatically better. Every step up costs disproportionately more stock for less added protection; the last few percent of service level is the most expensive cover you will ever buy.
Where safety stock sits in the chain
Safety stock is the first link. It feeds directly into your reorder point (the stock level that triggers a new order), which is your lead-time demand plus this buffer. From there you size how much to order with Economic Order Quantity (EOQ), and the cost of getting it wrong is what the Stockout-Cost Estimator puts a number on. Each tool hands its result to the next.
Terms used on this page
- Safety stock
- Buffer inventory held to absorb variability in demand and lead time, on top of the stock you expect to use during replenishment.
- Service level
- The proportion of replenishment cycles you intend to satisfy without running out. A planning target you choose, not a law.
- Lead time
- The elapsed time from placing a replenishment order to the stock being available to sell or use.
- Reorder Point (ROP)
- The on-hand level that triggers a new order: average lead-time demand plus safety stock.
- Standard deviation (σ)
- A measure of how much a quantity typically swings around its average. Larger σ means more variability and a bigger buffer.
Common questions
What is safety stock?
Safety stock is buffer inventory you hold on top of the stock you expect to use during a replenishment cycle. It absorbs the gap between forecast demand and actual demand, and between expected and actual lead time, so a normal swing in either does not push you into a stockout.
How is safety stock calculated?
This tool uses the standard statistical formula that accounts for variability in both demand and lead time: safety stock equals the service-level Z-factor multiplied by the square root of (lead time times demand variance, plus average demand squared times lead-time variance). If your lead time never moves, the second term drops out and it reduces to Z times demand standard deviation times the square root of lead time.
What service level should I choose?
The service level is the share of replenishment cycles you intend to get through without running out, and it sets the Z-factor in the formula. Higher cover is not automatically better: each step up costs disproportionately more stock for less added protection, so the last few percent of service level is the most expensive cover you will ever buy. Choose it per item, weighing the cost of holding stock against the cost of a stockout.
Does safety stock depend on lead-time variability?
Yes. When your lead time itself swings, that uncertainty often drives more of the buffer than demand variability does. Tightening a supplier's lead time, or making it more reliable, usually cuts safety stock faster and more cheaply than improving demand forecasts. Set lead-time variability to zero only if your lead time genuinely never moves.
Next in the chain
This is your starting number, not your final answer. Validate it against your own lead times, costs, and demand before you act on it.