The formula
Days inventory outstanding is how many days, on average, your stock sits before it sells. Average inventory divided by cost of goods sold, times 365. That is the whole formula: DIO = average inventory ÷ cost of goods sold × 365. On the worked example — $150,000 of average inventory and $900,000 of cost of goods sold — the DIO is 60.83 days. Your stock sits for 60.83 days before it sells, and your cash is parked in the warehouse for those 60.83 days.
Step 1 — Find the average inventory and cost of goods sold
Start with the average inventory, from the balance sheet. Average inventory is (opening inventory + closing inventory) ÷ 2. Then find the cost of goods sold, from the income statement. The DIO uses ONLY the cost of goods sold — never revenue, because revenue is what you sell, not what the stock cost you, and the markup was never sitting on the shelf. On the worked example the average inventory is $150,000.00 and the cost of goods sold is $900,000.00. Use the same period for both numbers: a year of cost of goods sold against a year of inventory, not a quarter against a year.
Step 2 — Work out the DIO
Divide the average inventory by the cost of goods sold, then multiply by 365: how many days the stock on hand lasts at the current selling rate. On the worked example it is $150,000.00 ÷ $900,000.00 × 365 = 60.83 days. That is the DIO: cash sits in stock for 60.83 days before the stock is sold. The same idea in reverse gives inventory turnover: 365 divided by the DIO, so a 60.83-day DIO is about 6 turns a year.
Step 3 — Check the result
Grocery and fast fashion often run 20 to 40 days, general retail 40 to 70 days, manufacturing 60 to 120 days, car dealers and heavy machinery longer because each unit is expensive and slow to move. On the worked example the DIO is 60.83 days, so the business sits at the top of the general-retail line and at the bottom of the manufacturing line — ordinary for a maker, long for a shop. A DIO near zero means stock sells almost as soon as it arrives, and the business barely waits for its cash at all.
Three worked examples
The headline example: $150,000.00 of average inventory and $900,000.00 of cost of goods sold. The days inventory outstanding is $150,000.00 ÷ $900,000.00 × 365 = 60.83 days. The target example: how much stock must the business hold for a 100.00-day DIO. The average inventory needed is 100.00 × $900,000.00 ÷ 365 = $246,575.34 — far more than the $150,000.00 it holds now, which shows how much more stock it would have to carry to stretch the DIO to 100 days. The reverse example: the smallest whole-dollar inventory for a 70.00-day DIO. It is $172,603; at $172,602 the inventory is $1 short of the $172,603 needed. Use the days inventory outstanding calculator for your own numbers.
One reverse to check your answer
The smallest whole-dollar average inventory for the DIO to reach 70.00 days is $172,603 — at $172,603 the DIO is 70.00 days; at $172,602 the inventory is $1 short of the $172,603 needed. The average inventory needed for a 100.00-day DIO is $246,575.34. If your own numbers give a DIO far from the one you expected, work the formula backwards: a wrong answer is usually a wrong input, and the most common mistake is using total revenue where the formula needs cost of goods sold, or closing inventory where it needs the average.
What average inventory is, and why it is an average
Average inventory is (opening inventory + closing inventory) ÷ 2: the stock at the start plus the stock at the end, divided by two. A single snapshot misleads: a business that stocks up in December and sells through in January shows huge inventory in December and small inventory in February. The average is steadier, because stock levels swing through the year. The same averaging idea applies to receivables: use (opening + closing) ÷ 2 for both legs of the operating cycle, and use the same period for the cost of goods sold and the net credit sales. If you only have the closing inventory, say so when you quote the DIO — it is a rough DIO, not a clean one, and a single big delivery can move it by weeks.
What a high DIO means — and what a low DIO means
A high DIO means cash is parked in the warehouse: you paid for goods that have not sold yet, and the business usually needs more working capital. A low DIO means cash comes back fast, and the business needs less working capital. A DIO near zero is the best of all — stock sells almost as soon as it arrives — but a DIO that is too low can mean stockouts, lost sales and rushed reorders. The goal is a healthy DIO, not the shortest possible one. Compare within an industry, not across them: a grocer and a car dealer with the same DIO are in very different positions, because a car dealer holds a few expensive units and a grocer holds many cheap ones.
What to do when the DIO is long
A 60.83-day DIO means each dollar of stock takes about two months to come back as cash. Shorten it by ordering smaller batches more often, so stock arrives as it sells instead of piling up in advance. Clear slow lines with discounts instead of letting them sit: a slow line at a discount is cash back, and the same line at full price is still a shelf full of stock. Stop reordering what does not sell, and check the slow movers every month instead of every year. The inventory the business must hold for a 100.00-day DIO is $246,575.34 — far more than the $150,000.00 it holds now — which shows how much cash would be tied up if the DIO stretched to 100 days. The other lever is the cost of goods sold: selling more of the same stock shortens the DIO without touching the shelves at all.
How DIO differs from the operating cycle
The DIO is one leg of the operating cycle: the operating cycle is days inventory plus days sales, so 60.83 + 91.25 = 152.08 days on the worked example, where the 91.25 days is the days sales outstanding — how long customers take to pay. The cash conversion cycle is the operating cycle minus days payable: 152.08 − 73.00 = 79.08 days, where the 73.00-day gap is exactly the days payable outstanding. The DIO is the picture before customers pay; the operating cycle is the picture before suppliers are paid; the cash conversion cycle is the picture after. Inventory turnover is the stock leg in different units: it tests how many times a year the stock sells through, and a 60.83-day DIO is about 6 turns a year. See the operating cycle calculator and the guide to calculating the operating cycle, and the guide to calculating days sales outstanding.