What is Days Inventory Outstanding (DIO)?
DIO measures how many days, on average, a product sits in inventory before it is sold. It is an efficiency metric. The lower the number, the faster inventory moves and the less cash is tied up in stock.
A DIO of 30 means inventory sells within 30 days on average. A DIO of 90 means it takes three months. That is three months of cash sitting on a shelf instead of working in the business.
The formula is:
DIO = (Average inventory ÷ Cost of Goods Sold) × Number of days in the period
Here is an example. A sporting goods shop has $30,000 in average inventory and $120,000 in quarterly cost of goods sold over 90 days. DIO = ($30,000 ÷ $120,000) × 90 = 22.5 days. That is a healthy number for most product businesses.
DIO matters because inventory is not free to hold. Storage costs, insurance, spoilage, and the risk of stock becoming outdated all add up. The longer the inventory sits, the more it costs the business beyond the purchase price.
How to measure your DIO using this calculator?
The calculator needs three inputs:
1. Beginning and ending inventory
Add the opening and closing inventory values for the period, then divide by two. This gives the average inventory. Using a single snapshot can be misleading, especially if stock levels shift significantly during the period.
2. Cost of goods sold (COGS)
This is what the business paid to produce or purchase the inventory it sold during the period. Find it on the income statement. Do not use revenue here. Revenue includes the markup. COGS reflects the actual cost of the stock, which is what this calculation requires.
3. Number of days in the period
Use 90 for a quarter or 365 for a full year. Be consistent. A quarterly DIO should only be compared to other quarterly figures.
What does your DIO result tell you?
A low DIO means inventory moves fast. Less cash is tied up, and the business is generally running efficiently. A high DIO means the stock is sitting longer than it should. That is worth investigating.
DIO is also one part of the cash conversion cycle, which measures how long it takes a business to turn inventory into cash. A lower DIO shortens that cycle, which means cash comes back into the business faster.
The right DIO depends on the industry. A grocery store might run a DIO of 10 to 15 days. A furniture retailer might run 60 to 90 days. Comparing your DIO to your own prior periods and your industry range gives a more useful read than any single benchmark.
A rising DIO over several periods is a warning sign. It could mean demand is slowing, purchasing is outpacing sales, or certain products are not moving. Any of these situations ties up cash and increases holding costs.
If a specific product line is driving a high DIO, that is worth addressing separately. Blanket reordering policies often mask slow-moving stock that needs to be discounted, returned to the supplier, or discontinued.