How to set a critical stock level: minimum stock calculation and safety stock formula

The purchasing officer has entered fifty as the critical level for every product, because the number is easy to remember. A fast-moving product drops to fifty in three days, and the shelf is empty before the supplier arrives. Another product that has not moved for months never drops to fifty and just sits in the warehouse. The same number produces a late alarm for one product and an unnecessary one for another. For a critical level to work, it has to depend on how fast the product is used and how long an order takes to arrive. In this article we show, with a hypothetical example, how the critical stock level is set and how minimum stock and safety stock are calculated with simple formulas.

The critical stock level is the quantity at which a product’s stock reaches the point where it must be reordered; it is defined so that stock does not run out before the supplied goods arrive. In the literature it is also called the reorder point. If you order when the product drops to this quantity, there will still be enough stock on the shelf to cover daily usage until the new batch arrives.

What does critical stock mean, and how is it different from minimum stock and safety stock?

The three terms are often confused. Although usage varies from company to company, the following distinction is common:

How do you calculate the minimum stock level? Step by step

1. Find average daily usage. Add up the outgoing movements for a recent period and divide by the number of days. Use calendar days, not only the days on which the product was used.
2. Determine maximum daily usage. Note the quantity on the day with the most outgoing stock in the same period; assess exceptions such as campaigns or one-off large orders separately.
3. Measure the lead time. Find the time between the day the order is placed and the day the goods receipt is recorded in the warehouse, by supplier if possible.
4. Calculate safety stock. Multiply the difference between maximum and average daily usage by the lead time.
5. Write down the critical level. Multiply average daily usage by the lead time and add the safety stock.
6. Enter the level on the stock card and set a review date. Update the level whenever usage or lead time changes.

Safety stock formula: a hypothetical calculation example

The figures below are entirely hypothetical; they were chosen only to show how the calculation works. The formulas were applied to three products.

In the example, the critical level of slow-moving product B is close to half that of product A because its lead time is long; but C, with the shortest lead time, carries the highest level because its daily usage is high. That is why, instead of entering the same number for every product, you need to calculate by product and supplier.

Let us follow the flow of product A. When stock drops to sixty units, the critical level, an order is placed. If an average of eight units goes out per day over the next five days, forty units are used and twenty remain; when the new batch arrives, the full safety stock is still on the shelf. Even if usage rises to twelve units on one day, the buffer covers it. If the supplier ships two days late, another sixteen units are used and stock drops to four; this is why measuring lead time is the most critical input of the calculation. After calculating the level, trying such short “what if” scenarios once shows whether the number is reasonable.

Which products should you calculate first?

In a business with hundreds of items, calculating levels for all of them with the same care is not realistic. A common method is to split products into three groups and set priorities accordingly:

• Critical group. Products whose stockout stops production, sales or service. Calculate the level by supplier and review it often.
• Middle group. Products used regularly but with available alternatives. Apply the formula once and check periodically.
• Low group. Low-value products or products that can be sourced quickly when needed. A simple lower limit and a visual check are usually enough.

Situations that distort the calculation

• Seasonality. For products whose summer and winter usage differ, split the level by period.
• Variable lead times. If the supplier sometimes ships later than expected, basing the calculation on the longest lead time rather than the average is safer.
• Minimum order quantity. If the supplier does not sell below a certain quantity, the critical level and the order quantity should be considered separately.
• Single-supplier risk. For products without an alternative, keeping a higher safety stock may make sense.
• Products that stop production. For items whose stockout would stop the line, looking at average usage is not enough; a higher, risk-based level is set.
• Campaigns and bulk orders. One-off large withdrawals inflate the average; filter them out before you run the calculation.

What do you do when a product reaches the critical level?

1. The person who receives the alert checks the product’s current stock and whether there is an open order; if a batch is already on its way, no second order is placed.
2. The current delivery time is confirmed with the supplier; if it has become longer, a decision is made on whether to dip into safety stock.
3. The order quantity is set: the quantity needed for the next period, the supplier’s minimum order quantity and warehouse capacity are considered together.
4. The order is recorded and the expected delivery date is noted, so a delay is noticed early.
5. When the goods arrive, the receipt is recorded the same day; stock rises above the level again.

Stock tracking and critical levels in Excel: when is it enough, and when does it fall short?

The formulas above are easy to set up in an Excel sheet: columns for usage, lead time and maximum usage, plus a critical level column. The problem is not the calculation but keeping it current. Usage changes every day, but the figure in the sheet is usually updated by hand; stock sits in one place and the level in another, and nobody is alerted when a product drops below the level. A spreadsheet is enough for calculating the level but falls short for alerts.

How do you see products that reach the critical level in KurumsalPortal?

The Inventory & Warehouse Management module does not calculate the critical level for you. The levels you calculate are monitored, and products that drop to them become visible. Because receipts, issues and transfers are kept in a single movement ledger, current stock is read from the record; across multiple warehouses you can see how much of which product is where on the same screen and assess it together with the stock report. This does not remove the need to recalculate levels by hand; but it means noticing a product below its level no longer depends on the habit of checking a spreadsheet.

Purchasing, warehouse and finance teams work from the same record. If you want to calculate your levels, see the products that reach the critical level in the record and order accordingly, you can review the flow on the Inventory & Warehouse Management page and the KurumsalPortal overview for other processes.