Days Inventory Outstanding (DIO)

Days inventory outstanding (DIO), also called days sales of inventory or days in inventory, is a working-capital metric that shows the average number of days a company holds its stock before selling it. It compares the value of inventory on the balance sheet with the cost of goods sold over a period. Manufacturers, retailers, distributors and their lenders use it to judge how efficiently goods move through the business.

DIO is the first stage of the cash conversion cycle, which adds DIO to days sales outstanding and subtracts days payable outstanding. It is closely related to the inventory turnover ratio: DIO is simply the number of days in the period divided by turnover. Unlike the other two measures, DIO is not about anyone else's payment behaviour; it reflects the company's own buying, production and selling, which is why it is a core measure in working-capital optimization.

Imagine a bookshop. Every book on the shelf was paid for with the shop's money, and that money comes back only when someone buys the book. A bestseller might sit for a week; a niche poetry collection might sit for a year. DIO is the average waiting time across the whole shop.

A short wait means cash returns quickly and the shop can restock with fresh titles. A long wait means money is stuck on the shelves, the shop pays rent for space used by slow sellers, and some books may never sell at full price. Shop owners therefore try to keep enough stock that customers find what they want, but not so much that cash sits idle.

The standard calculation is:

• $\text{Average inventory}$ is usually the opening plus closing inventory for the period, divided by two. Period-end inventory alone is also used.
• $\text{Cost of goods sold}$ is the cost of the products sold during the period, taken from the income statement. Revenue is not used, because inventory is carried at cost.
• $\text{Number of days}$ is the length of the period, typically 365.

A wholesaler starts the year with inventory of 2.0 million and ends it with 2.4 million, so average inventory is 2.2 million. Cost of goods sold for the year is 10.95 million. DIO is 2.2 divided by 10.95, about 0.201, multiplied by 365, which gives roughly 73 days. Inventory turnover is 10.95 divided by 2.2, about 5 times a year, and 365 divided by 5 also gives 73 days.

• Product type. Fresh food has to sell within days; machinery, jewellery and spare parts can sit for months.
• Production cycle. Inventory includes raw materials and work in progress as well as finished goods, so long manufacturing processes raise DIO.
• Supply-chain strategy. Just-in-time and lean manufacturing aim to keep DIO low, while firms worried about disruptions build safety stock and accept a higher figure.
• Demand forecasting. Over-ordering ahead of a season that disappoints leaves stock unsold and DIO rising.
• Seasonality. Retailers build stock before peak periods, so a reading taken just before the holidays will look much worse than one taken just after.

Inventory has to be paid for before it earns anything, so a high DIO increases the need for working capital funding. Companies often fund stock through a revolving credit facility whose borrowing base includes a percentage of eligible inventory, or through dedicated inventory financing. Commodity traders and farmers may borrow against goods held in a licensed warehouse through warehouse receipt financing. Lenders usually lend less against slow-moving or obsolete stock, so a rising DIO can shrink the amount a company can borrow.

The global supply shocks of 2021 and 2022, visible in measures such as the Global Supply Chain Stress Index, showed the trade-off clearly. Many firms that had run very lean inventories could not meet demand, and some deliberately raised DIO afterwards to build resilience, accepting higher funding costs in return.

DIO varies widely by sector. Grocery retailers commonly show figures of a few weeks, consumer electronics and apparel retailers a few months, and aircraft, pharmaceuticals or luxury goods makers well over 100 days. Comparisons are meaningful only against the company's own history and close peers.

• Accounting method matters. Inventory valued under first-in, first-out (FIFO) or weighted average cost gives different figures from last-in, first-out (LIFO), which US GAAP permits but IFRS (through IAS 2) does not. This can make cross-border comparisons misleading.
• Write-downs flatter the ratio. Writing obsolete stock down to net realisable value reduces inventory and lowers DIO without any improvement in sales.
• Averages hide detail. A healthy overall DIO can mask a pile of unsellable items, so analysts also look at inventory ageing reports.
• Low is not always good. Very low DIO can mean frequent stock-outs and lost sales.

For businesses that buy on credit, DIO should also be read against DPO: if suppliers are paid after the goods have already been sold, the stock has in effect been funded by the suppliers rather than the company.

• IFRS Foundation
• Financial Accounting Standards Board (FASB)