Safety stock: what it is and how merchants size it
Safety stock is the extra inventory you hold above expected demand so a sales spike or a late shipment does not empty the shelf. Sizing it is arithmetic: you take how fast the SKU sells, how long the supplier takes, and how wrong both of those numbers get on a bad week.
What safety stock is
Safety stock is the quantity of a SKU you deliberately hold beyond your expected sales during a supplier's lead time, so that variation in either demand or delivery does not turn into a stockout.
Expected demand is an average. Averages are wrong most days. A SKU that sells twelve units a day sells eight on a quiet Tuesday and twenty-two the week a creator mentions it. Lead time behaves the same way: the supplier who normally ships in fourteen days ships in twenty-one when a factory is backed up. Safety stock is the number you hold to absorb both of those without going to zero.
One common method, worked
A straightforward approach compares your worst case against your normal case:
(max daily sales x max lead time) minus (average daily sales x average lead time)
Say a hero SKU averages 12 units a day, peaked at 18 a day over the last quarter, normally arrives in 14 days and has been as slow as 21 days. That gives (18 x 21) minus (12 x 14), which is 378 minus 168, so 210 units of safety stock. That is a lot of cover, because it assumes the worst week and the worst shipment land together.
The simpler alternative is to pick a number of days you want to be covered for and multiply by velocity. Five days of cover on the same SKU is 12 x 5, so 60 units. Less protection, far less cash on the shelf. Both are defensible. What is not defensible is holding no buffer and calling the average a plan.
The inputs and where they come from
| Input | Example value | Where you get it |
|---|---|---|
| Average daily sales | 12 units | Units sold over a chosen window, divided by the days in it |
| Peak daily sales | 18 units | The busiest comparable days in recent history, not an all-time spike |
| Average lead time | 14 days | Your own purchase orders: order date to goods received |
| Worst lead time | 21 days | The slowest delivery you have actually seen from that supplier |
| Safety stock | 210 units | (18 x 21) minus (12 x 14) |
How much cover is the right amount
There is no single correct figure, because safety stock is a trade. Every unit of buffer is cash sitting still, warehouse space, and risk of markdown if the SKU fades. Every unit you skip is a chance of an empty product page. Merchants who catch a reorder point early can avoid the expensive version of that problem, but the size of the buffer is a judgement about which mistake you would rather make on that SKU.
A few practical rules of thumb hold up:
- Give the biggest buffer to SKUs with long lead times and lumpy demand. Those are where the average lies to you most.
- Give a small buffer, or none, to slow, predictable SKUs you can reorder quickly. The cash is better spent elsewhere.
- Revisit the numbers when a supplier's performance changes. A buffer sized on last year's lead time is a guess about this year's.
- Do not size the buffer off a single freak day. One viral week will inflate the figure for months.
If you are working out the order date rather than the buffer size, the sibling page on when to reorder covers turning velocity and lead time into a date you can act on.
How safety stock relates to your reorder point
Safety stock is a quantity. A reorder point is a trigger level. The reorder point is the demand you expect during the lead time plus whatever safety stock you decided to hold, and when stock on hand falls to that level, it is time to place the order.
Restoc calculates a reorder point per SKU from a configurable sales velocity window of 7, 14 or 30 days and the lead time you enter for that SKU. It does not compute a separate safety stock figure for you today, and we would rather say that plainly than imply otherwise. Merchants who want cover built in usually enter a lead time slightly longer than the supplier's typical one, which pushes the trigger level up by roughly the days of cover they wanted.
Related terms
- Lead time. The days between placing a purchase order and having sellable stock. Your number, from your own orders, not the supplier's brochure.
- Sales velocity. Units sold per day over a chosen window. A shorter window reacts faster to a trend and is noisier; a longer one is steadier and slower.
- Lead time demand. Average daily sales multiplied by lead time. The expected sales you must cover before the next delivery lands.
- Days of inventory remaining. Current stock divided by sales velocity. The plain-language version of how long you have left.
- Stockout. Selling out of a SKU that customers still want. The outcome all of the above arithmetic exists to prevent.
Questions
How much safety stock should I hold?
Enough to cover the gap between your normal week and a bad one. Worked on the max-minus-average method, a SKU selling 12 a day with a 14-day lead time that can peak at 18 a day and slip to 21 days needs 210 units. If that number ties up too much cash, choose a number of days of cover instead, for example five days at 12 a day is 60 units, and accept the smaller margin of error.
What is the difference between safety stock and a reorder point?
Safety stock is a buffer quantity. A reorder point is the stock level at which you place the order, and it equals expected demand during the lead time plus that buffer. One is how much you keep spare, the other is when you act.
Do I need safety stock on every SKU?
No. Slow, predictable products you can restock quickly rarely justify holding extra cash on the shelf. Long lead times and volatile demand are where the buffer earns its place.
Does Restoc calculate safety stock for me?
Not as a separate number. Restoc calculates a reorder point per SKU from a sales velocity window of 7, 14 or 30 days and the lead time you enter for each SKU. If you want cover included, enter a lead time a few days longer than the supplier's typical one.
Where do I get an accurate lead time?
From your own purchase order history: the days between placing the order and having stock you can sell, including receiving and putaway. Use the slowest real delivery you have seen as your worst case rather than an estimate.
Should I recalculate safety stock often?
Recalculate when either input moves, meaning demand shifts into a new normal or a supplier's delivery times change. A buffer sized on last season's numbers protects last season's store.