Sellevate
Free tool from Sellevate · USD only

Safety stock calculator

Average demand and average lead time hide the risk — it's the worst weeks and the slowest shipments that cause stockouts. This works out the safety stock buffer you need to cover that variability, feeding straight into a reorder point.

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What your result means

Your result is the number of extra units to keep in stock so you don't run out during a bad restock cycle, when sales run hot and the shipment is late at the same time.

Add it to your expected demand during lead time to get your reorder point. Safety stock doesn't get used up on a normal cycle; it's there for the bad ones.

How the safety stock formula works

This calculator uses the basic (max-minus-average) method:

Formula

Safety stock = (Max daily sales × Max lead time) − (Avg daily sales × Avg lead time)

Max daily sales
your peak daily sales in a typical period, in units
Max lead time
the longest a restock has realistically taken, in days
Avg daily sales / Avg lead time
what a normal day and a normal restock look like

It's simple and needs no statistics, but it's cautious: it assumes your worst sales and your worst delay happen together.

The service-level method

If you track daily sales history, the service-level formula sizes the buffer to a target probability of not running out:

Service-level safety stock

Safety stock = Z × σ(daily demand) × √(Lead time in days)

Z
the service-level factor: about 1.28 for 90%, 1.65 for 95%, 2.33 for 99%
σ(daily demand)
standard deviation of daily unit sales
Lead time
average lead time in days

This version assumes lead time is steady and demand varies. It usually gives a smaller, better-targeted buffer than the basic method.

Worked example

Worked exampleExample numbers

These match the calculator's default inputs.

InputValue
Peak daily sales20 units
Worst-case lead time21 days
Average daily sales12 units
Average lead time14 days
  1. Bad cycle demand = 20 × 21 = 420 units
  2. Normal cycle demand = 12 × 14 = 168 units
  3. Safety stock = 420 − 168 = 252 units

For comparison, with the service-level method at 95% (Z = 1.65), a daily demand standard deviation of 4 units and the same 14-day average lead time: 1.65 × 4 × √14 = 1.65 × 4 × 3.74 ≈ 25 units. The gap shows how conservative the basic method is when worst-case sales and lead time rarely coincide.

How to keep safety stock lean

  • Shorten and steady your lead times. Lead time variability drives safety stock as much as demand does.
  • Set different levels by product. Hold more on best-sellers where a stockout is costly (see the stockout revenue loss estimator), less on slow or low-margin items.
  • Use realistic peaks. One freak sales day shouldn't set your maximum. Use a typical busy day.
  • Review it with your reorder point. Recalculate both when sales or supplier performance changes.

FAQ

How do you calculate safety stock?

The simplest method is (maximum daily sales × maximum lead time) − (average daily sales × average lead time). It covers the gap between a normal restock cycle and a bad one. A more precise method uses a target service level and the variability of your daily demand.

Is buffer stock the same as safety stock?

Mostly, yes. Both mean extra inventory held to avoid running out. Some people use buffer stock more broadly to include stock built up ahead of a known event, such as a promotion or peak season, while safety stock covers unexpected variation.

How much safety stock is too much?

If you rarely dip into it, it's probably too high. Safety stock is cash sitting on a shelf, so check how often you actually use it. Products with steady demand and reliable suppliers need much less than volatile ones.

What service level should I use?

Higher service levels need disproportionately more stock. Many stores use a high level, such as 95% or more, for best-sellers where a stockout is expensive, and a lower level for slow or low-margin products.

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