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The safety stock formula
Safety stock is the extra stock you hold so an ordinary bad week does not empty your shelves. Too little and any small delay wipes you out. Too much and your cash is sitting in a warehouse doing nothing. Here is how to pick the amount on purpose.
By Fazeel Tanveer, Founder · Last reviewed 25 August 2026
What safety stock is
Safety stock is the buffer you hold so an ordinary bad week does not empty the shelf. Hold too little and one late delivery costs you sales. Hold too much and you have tied up cash in stock that just sits. This page gives a rule of thumb you can apply today plus the statistical version that uses demand variability against a service level you choose.
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What safety stock actually protects you from
Safety stock covers two separate risks. Which one you are exposed to changes how much you need.
- You sell more than expected. A press mention, a post that takes off, a competitor running out before you do.
- The restock takes longer than planned. Production slips, the shipping is late, customs holds your container.
A product that sells steadily from an unreliable overseas supplier needs cover for the second. A product that suddenly takes off with a dependable local supplier needs cover for the first. Anything exposed to both needs considerably more than either on its own.
The simple way to work it out
This is good enough for most products and far better than holding no buffer at all.
Safety stock = lead time demand × a buffer percentage
| Product profile | Buffer |
|---|---|
| Sells steadily, reliable supplier | 20 to 25 percent |
| A bit unpredictable either way | 30 to 50 percent |
| Sales jump around or the supplier is unreliable | 50 to 100 percent |
| An item you cannot afford to be without | 100 percent or more |
Selling 8 a day with a 21 day restock means you sell 168 units while you wait. A 33 percent buffer on top of that is 56 units of safety stock.
The precise version for your best sellers
Worth the effort on expensive products, where holding too much and holding too little both cost you real money.
Safety stock = Z × standard deviation of demand during lead time
Z is a multiplier that depends on how often you are willing to run out:
| How often you want stock | Z | What that means |
|---|---|---|
| 90 percent | 1.28 | Out of stock in roughly one cycle in ten |
| 95 percent | 1.65 | One in twenty |
| 98 percent | 2.05 | One in fifty |
| 99 percent | 2.33 | One in a hundred |
The cost is the part to watch. Going from 95 to 99 percent is not a small step. It roughly doubles the stock you hold to buy four percentage points of cover. Worth it on your best seller. A waste of money on an accessory nobody misses.
Safety stock is cash sitting on a shelf
Every unit of safety stock is money you have swapped for peace of mind. It costs you: cash tied up, storage, insurance, stock going out of date and the discount you take to clear it at the end of a season.
In theory the right level is where one more unit of buffer costs exactly what it saves you. In practice nobody works that out. What you can do is choose on purpose: a big buffer on the products where running out is expensive, a small one where it is merely annoying.
Using one percentage across everything is the same mistake as using one threshold across everything. You overspend on the products nobody misses and leave the ones that matter exposed.
Set your alert above your safety stock
Safety stock is not the level you want to hear about. It is the reserve you are trying not to touch at all.
Your alert belongs at the reorder point, which is the units you sell during a restock plus your safety stock. That way the order goes in while the buffer is still untouched. See the reorder point formula.
Protect the buffer you paid for
Safety stock only works if something spots the problem before you start eating into it. Stockwell keeps your reorder point for every SKU and emails your team the moment stock drops past it.
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Common questions
How much safety stock should I hold?
For steady products with a reliable supplier, 20 to 25 percent of what you sell during a restock. For unpredictable sales or an unreliable supplier, 50 to 100 percent. For items you cannot be without, more. The precise version multiplies a service factor by how much your sales swing during a restock.
What service level should I target?
95 percent is the usual starting point. Only go to 98 or 99 on products where running out really costs you, because the stock you hold roughly doubles between 95 and 99 percent for a small gain.
Is safety stock the same as a reorder point?
No. Safety stock is the reserve itself. The reorder point is what you sell during a restock plus that reserve, so it is the bigger number. You order at the reorder point so the reserve never gets touched.