How to Calculate Inventory Coverage

How to Calculate Inventory Coverage

When you buy merchandise for your business, one of the most useful questions you can ask yourself is very simple: how many days will the stock I have in the storeroom last? Inventory coverage answers exactly that question. It is a figure that connects what you have available with the speed at which you sell it, and it tells you, in days, how long your business can keep selling before it needs to place a new order with the supplier.

It is a simple calculation, but a highly revealing one. With it you may notice, for example, that a product you considered well stocked will actually run out in less than a week, or that another one has been sitting on the shelf for months, with money invested in it that is not doing any work. This article explains what coverage is, how to calculate it with a step-by-step example, and how to use it to buy better.

What inventory coverage is

Inventory coverage, also called days of stock on hand, is the estimated time during which the current stock of a product can support the pace of sales the business has today. If you currently have 120 units of a product and you sell 6 units per day on average, your inventory will last 20 days. In other words: if no new merchandise arrived, you would have products to sell for twenty days.

It is worth being precise about what it measures. Coverage does not tell you how much the inventory is worth or whether the purchase was a good one; it tells you how long what you have will last at the current pace of sales. That is why it is an anticipation tool: by knowing it before the product runs out, you can calmly decide when to order, how much to order, and which product to prioritize on the next purchase.

It is also worth remembering that it is a projection, not a promise. If sales go up or down, coverage changes. A large customer order, a peak season, or a slow week can move the number from one day to the next. That does not make it less useful: it is precisely why you review it often, like a snapshot that is updated every week.

The inventory coverage formula

The formula is simple:

Coverage (in days) = Current stock ÷ Average daily sales

It is calculated product by product, using the same units on both sides of the division. If stock is measured in units, average daily sales must also be in units. The result is expressed in days and can be rounded to work with whole numbers.

How to get the average daily sales figure

Average daily sales come from the product's sales history. You add up the units sold in a period and divide them by the number of days in that period. For example: if 180 units of a product went out in the last 30 days, average daily sales are 6 units (180 ÷ 30).

For the average to be reliable, the period must reflect the business's normal pace. Thirty days is usually a good starting point; sixty days works better when sales are irregular or the product moves slowly. If your business is strongly seasonal, compare the figure with the same period last year or calculate coverage by season, instead of using a yearly average that does not represent any real month.

Example: calculating days of coverage

To see it in practice, imagine a small grocery business that wants to review four products. The person in charge checks the stock on hand and calculates the average daily sales of each product using the last 30 days of movement. With those two figures, they apply the formula:

Rice is calculated by dividing 240 by 12: 20 days. The oil, 90 by 6: 15 days. The coffee, 36 by 9: 4 days. The soap, 150 by 5: 30 days. In a single table the business can already see where the urgency is: the coffee will run out in less than a week if it is not ordered soon, while the soap has enough merchandise for a full month.

How to interpret coverage

Reading the result is as important as calculating it. Low coverage means the product may run out soon: if the supplier takes longer to deliver than the inventory will last, the business runs out of stock and loses sales. High coverage means a lot of capital is sitting idle: the money invested in that merchandise could be buying other products or covering expenses, and the longer it sits, the greater the risk of expiration, damage, or obsolescence.

There is no single coverage number that is right for every product. The correct benchmark combines three factors:

• Supplier lead time. If a supplier takes 15 days to deliver, the coverage of that product should allow selling without risk during that period, plus a safety margin.
• The type of product. Perishable items and fashion products need short coverage; stable products with steady sales can tolerate wider coverage.
• The business's policy. Some businesses prefer to risk running short rather than tying up money; others prefer to always have extra stock. Coverage makes that decision visible.

The most practical way to interpret it is to compare each product's coverage with its replenishment time. A product with 10 days of coverage and a supplier lead time of 20 days is at risk right now, even if it looks well stocked. Another with 45 days of coverage and a 5-day lead time is tying up capital that could be used elsewhere.

How to use coverage to buy better

Coverage stops being a curiosity when it becomes the basis of your purchasing plan. Here is a simple method to apply it:

1. Keep your records up to date. Coverage is only reliable if stock levels and outgoing movements are properly recorded. With an up-to-date stock record that tracks the stock on hand and the sales history of each product, as Kardex Tauro allows, you can calculate the coverage of every item without relying on memory.
2. Calculate coverage regularly. A weekly or biweekly review is enough for most small businesses. Fast-moving products are reviewed more often than slow-moving ones.
3. Set a minimum coverage for each product. That target should be longer than the supplier's lead time, so merchandise never runs out while you wait for the order.
4. Rank the purchase by urgency. When it is time to order, buy first the products whose coverage is below the target and then the ones that are close to it.
5. Hold off on products with high coverage. If a product still has 40 days of merchandise, it can probably wait until the next buying cycle.

With this order, purchases stop being made "just in case" or because the supplier happens to be in the neighborhood, and start being made because the numbers call for it. Over time, the business buys less of what is left over and more of what is missing, and that shows in the cash flow.

Inventory coverage is one of those calculations that any small business can make with data it already has. You only need to know how much you have, how much you sell per day, and be willing to let those two numbers guide the next purchase.