Days of inventory coverage: the reorder point and safety stock

What inventory coverage is

A hundred units in stock mean little on their own: for an item that sells a hundred a day they are one day, for one that sells one a day they are more than three months. Coverage turns stock into days, item by item, so that items moving at different speeds can be compared.

That is why a fixed quantity threshold, such as «reorder below 50 units», usually works poorly across a wide range. A threshold in days reads the same way for every item and can be set directly against the supplier's delivery time.

Formula: forecast demand or past consumption

Coverage can be calculated looking forward, from forecast demand, or looking back, from average consumption over recent weeks. The backward-looking version is the most common because it uses data you already have, and when demand is stable the two numbers are close.

When demand changes, though, past consumption describes a pace that is gone. At the start of a season, backward-looking coverage looks longer than it really is and the reorder goes out late; at the end it looks shorter and you order goods that will sit on the shelf. Starting from the forecast helps you reorder at the pace that is coming: how to build one is covered in the guide to demand forecasting.

Inventory turnover and days inventory outstanding (DIO)

Inventory turnover tells you how many times stock is renewed in a period; DIO (days inventory outstanding) says the same thing in days. With annual cost of goods sold of 4.8 million euros and average inventory of 800 thousand euros, turnover is 6 and DIO is 365 ÷ 6 = 60.8 days. Together with DSO and DPO it makes up the cash conversion cycle, explained in the guide to DSO calculation.

Turnover and DIO look back, in value, across the whole warehouse: they serve the accounts. Coverage looks forward, in units, item by item: it serves whoever places the orders. A DIO of 61 days can be the average of items that run out in five days and others with a year's worth of stock, and that gap is what coverage by item brings out.

Reorder point

The reorder point is the stock level below which it makes sense to order: enough to cover demand during the lead time, the time between placing an order and receiving the goods, plus safety stock. In practice it is compared with the inventory position rather than physical stock alone: an item below its reorder point with an order already on its way usually does not need ordering again.

Safety stock: a simple rule and the statistical formula

The simple rule holds a few extra days of demand: it is easy to apply, but it treats a steady item and a volatile one the same way. The statistical formula, the textbook approach in inventory management, sizes the buffer on demand variability and on the service level, the probability of not running out during a replenishment cycle. The z factor comes from the normal distribution.

The last points of service cost more than the first, which is why the level is usually chosen per class of items. The basic formula assumes a fixed lead time: if the supplier delivers in 14 days on average with a standard deviation of 4, the full formula gives 1.65 × √(14 × 20² + 40² × 4²) = 1.65 × √31,200 ≈ 291 units at 95%, more than twice 123. Supplier variability often weighs more than demand variability.

Worked example

A distributor holds 1,800 units of an item. The forecast for the next 30 days is 1,200 units, 40 a day. The supplier delivers in 14 days and safety stock is 300 units, equal to 7.5 days of demand. Over the last 30 days 900 units were sold, 30 a day: the season is just starting. The figures are fictional.

Take the 14 days of lead time and the 7.5 days of safety stock away from 45 days of coverage and 23.5 are left, exactly the days before reordering. If demand runs to forecast, in 23.5 days stock falls to 860 units, the order goes out, and when the goods arrive 14 days later the 300 units of safety stock are still there. From past consumption alone coverage would look like 60 days, 15 more: enough to order late just as demand picks up.

ABC and XYZ classification: where to hold stock

ABC classification ranks items by the value they move; XYZ ranks them by how regular demand is, measured with the coefficient of variation (standard deviation of demand ÷ average demand, over weeks or months). Taken together, they help decide where to hold stock and at what service level.

Reading coverage item by item

Coverage is read against the lead time and the safety stock expressed in days. In the example the reorder threshold is 14 + 7.5 = 21.5 days.

Common mistakes

• Using annual average demand for seasonal items: it understates coverage out of season and overstates it as the season begins.
• Ignoring open purchase orders: you reorder an item already on its way, or count as available stock already promised to customers.
• Taking lead time without its variability: a supplier that delivers in 14 days on average but sometimes in 20 needs more safety stock than a punctual one.
• Giving every item the same safety stock as a percentage: too much on steady items, too little on volatile ones.

Many of these mistakes come from data kept in different places. In Muvia, sales and stock are copied every night from the ERP and the warehouse system, coverage in days is worked out every day for every item, and an alarm opens when it drops below the threshold and closes once the goods arrive, with who handled it written in the register: that is the Stock coverage and low-stock alarms use case. If your team has a forecasting model, coverage can start from forecast demand, as in Demand and stock forecasting.

Calculator

Enter stock on hand, forecast demand for the next 30 days, lead time and safety stock: the calculator returns coverage in days, the reorder point and the days left before reordering. The starting values are those from the example.