Inventory
Days inventory outstanding: formula and how to lower it
Days inventory outstanding (DIO) is the average number of days it takes your store to sell through its inventory. You calculate it by dividing average inventory by cost of goods sold (COGS) and multiplying by the days in the period. A lower DIO means stock turns into cash faster.
The days inventory outstanding formula
DIO = (Average inventory ÷ COGS) × Days in period
- Average inventory
- (Beginning inventory + Ending inventory) ÷ 2, valued at cost
- COGS
- cost of goods sold for the same period
- Days in period
- 365 for a year, about 90 for a quarter, 30 for a month
Both inventory and COGS must be at cost, not retail price. Mixing a retail inventory value with COGS inflates the result.
Use the same period for every input. If COGS covers a quarter, use inventory from the start and end of that quarter, and about 90 days.
Worked example: a year of DIO for a Shopify storeExample numbers
| Input | Value |
|---|---|
| Inventory at cost, January 1 | $90,000 |
| Inventory at cost, December 31 | $110,000 |
| COGS for the year | $500,000 |
- Average inventory = ($90,000 + $110,000) ÷ 2 = $100,000
- DIO = ($100,000 ÷ $500,000) × 365 = 0.2 × 365 = 73 days
On average, this store holds about 73 days of stock before it sells.
DIO vs inventory turnover
DIO and inventory turnover measure the same thing from opposite directions. Turnover counts how many times you sell through your average inventory in a period. DIO counts how many days one cycle takes.
Inventory turnover = COGS ÷ Average inventory DIO = 365 ÷ Inventory turnover
In the example above, turnover is $500,000 ÷ $100,000 = 5 times a year, and 365 ÷ 5 = 73 days, the same answer.
Turnover is easier to compare across years. DIO is easier to compare with things measured in days, like supplier lead times and payment terms.
Days sales of inventory, inventory days on hand: same idea
You'll see this metric under several names:
| Name | Abbreviation | Same as DIO? |
|---|---|---|
| Days inventory outstanding | DIO | — |
| Days sales of inventory | DSI | Yes |
| Days in inventory | DII | Yes |
| Inventory days on hand | DOH | Usually; sometimes based on current stock rather than average stock |
The one difference worth watching is which inventory figure goes in the top of the formula. Average inventory smooths out seasonal swings. Current inventory gives a snapshot of today, which is useful for spotting a build-up but jumps around more.
How to calculate DIO for a quarter or a month
Annual DIO hides seasonality. For a store that stocks up before Black Friday, a quarterly or monthly view shows when cash is really tied up.
Worked example: quarterly DIOExample numbers
| Input | Value |
|---|---|
| Average inventory at cost for Q3 | $100,000 |
| COGS for Q3 | $120,000 |
| Days in period | 90 |
DIO = ($100,000 ÷ $120,000) × 90 = 0.833 × 90 = 75 days
If Q3 DIO is 75 days while the full year averages 73, that's normal pre-season stocking. If DIO keeps rising quarter after quarter without a seasonal reason, stock is piling up faster than it sells.
DIO benchmarks by retail category
How many days of stock is normal depends heavily on what you sell. The most reliable public benchmark is the US Census Bureau's inventories-to-sales ratio, published monthly by kind of business.
It measures how many months of sales retailers hold in stock. Multiplying it by 30.4 (the average days in a month) converts it to days.
| Retail category (US) | Inventories-to-sales ratio | Days of sales in stock |
|---|---|---|
| Clothing and clothing accessories | 2.11 | about 64 |
| Building materials and garden | 2.15 | about 65 |
| Furniture, electronics and appliances | 1.60 | about 49 |
| General merchandise | 1.24 | about 38 |
| Food and beverage | 0.77 | about 23 |
| All retail, excluding auto dealers | 1.09 | about 33 |
Source: U.S. Census Bureau, Monthly Retail Trade survey, seasonally adjusted, July 2026, via FRED, Federal Reserve Bank of St. Louis.
Read these as a rough guide, for three reasons:
- They're sales-based, not COGS-based. Census divides inventory at cost by sales at retail price, so the days figure is lower than a true DIO. To compare with your own DIO, divide the benchmark by (1 − your gross margin). At a 40% gross margin, the clothing benchmark of about 64 days works out to roughly 107 days of DIO.
- They mostly reflect physical retailers. Census counts stock in stores and the warehouses that serve them. Online-only brands, especially ones that pre-order large seasonal batches, can run very different numbers.
- They're averages across thousands of businesses. Your product mix, supplier lead times and seasonality matter more than any category average.
The most useful comparison is still your own DIO over time. If it rises for several months in a row without a seasonal reason, stock is building up faster than it sells.
What a high DIO means for cash
Every day of DIO is cash sitting on your shelves. You've paid suppliers, but you won't get that money back until the stock sells.
Using the same example store, here's what cutting DIO from 73 to 60 days would mean, with COGS unchanged:
- Inventory needed at 60 days = $500,000 × 60 ÷ 365 = about $82,200
- Current average inventory = $100,000
- Cash freed up = about $17,800, with no change in sales
A high DIO also raises carrying costs: storage, insurance, shrinkage and the risk of products going out of date. Those costs land hardest on dead stock, the slowest-moving part of your inventory.
Cash Tied Up in Inventory CalculatorHow many days your cash sits as inventory before it becomes cash again.How to lower days inventory outstanding
You lower DIO by holding less stock for the same level of sales, without causing stockouts.
- Set reorder points from real data. Reorder when stock hits a level based on daily sales and lead time, not on a gut feeling or a fixed date. The reorder point calculator does the math.
- Size safety stock to actual variability. Too much buffer inflates DIO; too little causes stockouts. The safety stock calculator shows the buffer your numbers justify.
- Clear dead stock. Products that haven't sold in 90 days or more drag your average up. Bundle them, discount them or liquidate them, then stop reordering them.
- Order smaller batches more often where supplier minimums allow it. The EOQ calculator shows the order size that balances ordering and holding costs.
- Forecast by SKU, not by store. A store-level forecast hides which products are overstocked and which are about to run out.
FAQ
What is a good days inventory outstanding?
It depends on what you sell. US Census data for July 2026 shows clothing retailers holding about 64 days of sales in stock and food and beverage stores about 23, so one target can't fit every store. Compare yourself with your category, then track your own DIO over time against your supplier lead times.
Is a lower DIO always better?
No. A very low DIO can mean you're running too lean and stocking out, which costs sales and wastes ad spend. The aim is the lowest DIO that still keeps best-sellers in stock through your lead time, with enough safety stock to absorb normal swings in demand.
What's the difference between DIO and days sales of inventory?
Nothing, in practice. Days inventory outstanding, days sales of inventory (DSI), days in inventory and inventory days on hand all describe the same measure: how many days of sales your inventory represents. Some sources compute 'days on hand' from current stock rather than average stock, so check which inventory figure is used.
Should I use 365 or 360 days?
Use the actual number of days in the period you're measuring: 365 for a year, about 90 for a quarter, 30 for a month. Some finance teams use 360 for convenience. It matters less which you choose than using the same convention every time you compare.
Can I calculate DIO from Shopify reports?
Yes, if you track product costs. You need COGS for the period and inventory value at cost at the start and end of it. Shopify's reports can show cost of goods sold and inventory value when unit costs are entered for your products; without unit costs, neither number is reliable.