Safety stock is the buffer that absorbs the gap between an average week and a bad one. It is worst-case daily sales multiplied by worst-case lead time, minus average daily sales multiplied by average lead time. Enter your four numbers below. The result is what standing between you and a stockout costs.
Work out your safety stock
Nothing is sent anywhere. The calculation happens in this page.
Safety stock ≈ (worst-case daily sales × worst-case lead time) − (average daily sales × average lead time)
Reading the result
The result is the cost of your own uncertainty, expressed in units. A large number does not mean you are doing something wrong. It means your supplier is unpredictable, your demand is volatile, or both, and the buffer is what that unpredictability costs you in cash on a shelf.
Which is why the first response to a big number should be to look at the inputs rather than the output. Cutting worst-case lead time from 28 days to 24, by chasing earlier, reduces the buffer more cheaply than holding the extra stock.
A negative result means your worst case is not worse than your average, which means one of the two is wrong. Usually the worst case has been estimated optimistically.
When the number looks wrong
This calculation is the one most often fed bad inputs, because both worst-case figures are guesses unless you have looked them up.
- The number is enormous. Check the worst case. People tend to enter their single worst day ever rather than a realistic bad week, and one Black Friday does not set your year-round buffer.
- The number is zero or negative. Your worst case and average are the same, so either you have a very reliable supplier and steady demand, which is genuinely possible, or the worst case has not been thought about.
- It feels unaffordable. That is a real finding rather than a calculation error. The choice is holding it, tightening the supplier, or accepting a known stockout risk. Pretending the buffer is smaller is not one of the options.
Where the inputs come from
- Worst-case daily sales: a bad week you have actually had, not a hypothetical. Look at your highest normal week and use its daily rate.
- Worst-case lead time: the longest this supplier has actually taken, from your own records. If you do not have records, that is the more urgent problem.
- Average figures: the same numbers you would use for a reorder point, across a period without a sale or a stockout in it.
The derivation and a worked example are on the safety stock definition.
Common questions
How much safety stock should I hold?
Enough to cover the gap between a bad case and an average one, which this calculates. There is no percentage rule that works across products, because the right buffer depends entirely on supplier reliability and demand volatility.
What if I do not know my worst-case lead time?
Use the longest delivery you can remember from that supplier and treat the result as provisional. Then start recording delivery dates, because this number is unknowable without them and it drives everything else.
Is safety stock wasted money?
It is money converted into reliability. Whether that trade is worth it depends on what a stockout costs you: lost margin on the sale, plus the customer who buys elsewhere and does not come back.
Should every product have safety stock?
No. Slow movers with reliable supply often do not need any, and holding a buffer on a product that sells twice a month is cash sitting still for no reduction in risk.