# Safety stock calculator > Size the buffer that covers a busy week and a slow supplier. - Source: https://storestandards.com/tools/safety-stock-calculator/ - Kind: Calculator - Updated: 2026-08-15 - Author: Nia Chambers, Product, Store Standards --- ## The formula Safety stock is the gap between a bad month and a normal one. ``` safety stock = (worst daily sales × worst lead time) − (average daily sales × average lead time) ``` You are working out two pictures of the same period. One is the month where everything goes wrong. One is the month where nothing does. The difference between them is the stock that gets you through the first. ## A worked example On your busiest normal days you sell twenty units. On average you sell twelve. Your supplier usually takes fourteen days and once took twenty one. The bad case is twenty times twenty one, which is 420 units. The normal case is twelve times fourteen, which is 168 units. The difference is 252 units. That is your buffer, and at twelve a day it buys you about three extra weeks. ## What a good number looks like A buffer worth holding covers a real bad month. It does not cover the worst week in the shop's history, because stock that sits still is cash that sits still, and it can turn into [dead stock](https://storestandards.com/glossary/dead-stock/). Two honest inputs make this work. Use your busiest normal week, not your single best day. Use the slowest that supplier has actually been, not the slowest you can imagine. If the answer comes out at zero, you have entered the same figure twice. That means you have described a supplier who is never late and a shop that never gets busy. Neither exists. ## Where this goes next Safety stock on its own changes nothing. It works by raising the level at which you place the order, so take the answer and put it into the [reorder point calculator](https://storestandards.com/tools/reorder-point-calculator/). A buffer is also the cheapest fix for a supplier who misses dates. Before you raise it, check whether the real problem is a [stockout](https://storestandards.com/glossary/stockout/) caused by bad counts rather than by late deliveries. Buffering a counting problem just hides it at your own cost. ## What it asks for - **Units sold a day, worst case**: Your busiest normal day, not Black Friday. - **Units sold a day, average**: A normal month, divided by its days. - **Lead time in days, worst case**: The slowest this supplier has ever been. - **Lead time in days, average**: What the supplier normally takes. ## What it gives back - **Hold a buffer of** (units) - **Worst case demand** (units) - **Normal demand** (units) - **Extra days this buys you** (days) ## Questions **What is safety stock?** It is the stock you hold on top of what you expect to sell. It exists for the weeks when the shop is busier than usual and the supplier is slower than usual, which tend to be the same weeks. **Why does this ask for four numbers?** Because a buffer covers two risks at once. Sales can run high and delivery can run late. Asking for the normal and the worst of each is what separates a sized buffer from a guess. **What counts as a worst case?** The busiest normal week you have had in the last year, and the slowest that supplier has ever delivered. Do not use Black Friday. A buffer sized for one day a year is stock sat still for the other 364. **Is more safety stock always safer?** No. Every unit of buffer is cash on a shelf, and it ages. The point of sizing it is to hold enough to cover a bad month and not a penny more. **What do I do with the answer?** Put it into the reorder point calculator as your buffer. Safety stock on its own does nothing. It works by lifting the level at which you reorder.