Glossary - Products and inventory
Days inventory outstanding (DIO)
Days inventory outstanding (DIO) is the average number of days a business holds inventory before it is sold, calculated as average inventory at cost divided by cost of goods sold, multiplied by the days in the period. Ecommerce operators use it to see how long cash stays locked in stock before it comes back as a sale.
| Variable | Definition |
|---|---|
| Average inventory | Opening plus closing inventory value, divided by two, at cost. For a seasonal business, average monthly snapshots instead of two end points. |
| COGS | Cost of goods sold over the same period, ideally on a landed basis so the numerator and denominator are both at true cost. |
| Days in the period | 365 for a year, 90 or 91 for a quarter, 30 for a month. Match it to the COGS period. |
The days inventory outstanding formula is the mirror of inventory turnover: turnover is COGS ÷ average inventory, so DIO is simply days in the period ÷ turnover. The same measure is also called days sales of inventory (DSI) or days in inventory.
Worked example
On average, a dollar spent on stock waits 60 days to come back as a sale. Because this store pays suppliers on 30-day terms and receives card payouts within a couple of days, it funds 32 days of the gap from its own working capital. That is the cash conversion cycle, and DIO is almost always the largest part of it for a product business. Cut DIO to 45 days on the same sales and roughly A$45,000 of average inventory is released as cash: A$270,000 ÷ 90 × 15 days.
What is a good days inventory outstanding?
There is no honest universal number. DIO depends on the category (fashion with seasonal drops, consumables that replenish monthly, and furniture with long lead times all behave differently), supplier lead times and minimum order quantities, and how much breadth you choose to carry. A brand importing by sea needs more days on hand than one buying from a local wholesaler weekly.
What good looks like is a DIO that matches your replenishment cycle plus a deliberate safety buffer, and a trend that is flat or falling while stockouts stay rare. A falling DIO bought with more stockouts is not an improvement; it is lost margin moved off the balance sheet.
DIO vs related metrics
| Metric | What it tells you | How it differs |
|---|---|---|
| Inventory turnover | How many times stock sells through in a period | The same information inverted: a count of turns rather than a count of days. |
| Days of cover | How long current stock lasts at the forecast sales rate | Forward-looking and per SKU, used to reorder; DIO looks back at the whole book, used to judge cash. |
| Cash conversion cycle | Days from paying suppliers to collecting from customers | DIO is one input, alongside days to collect and supplier payment terms. |
| GMROI | Gross margin earned per dollar of inventory | Adds margin: a slow SKU with fat margin can still earn its shelf space. |
Common mistakes
- Mixing cost and retail values. Inventory at retail divided by COGS at cost inflates DIO by your markup. Keep both sides at cost.
- Using only closing inventory. One snapshot on a quiet day, or straight after a big delivery, distorts the result. Average across the period.
- Reading the store-wide average as the answer. A healthy 60 days can hide fast movers at 20 and a dead stock tail at 400. Compute DIO by category or SKU.
- Mismatching periods. Quarterly COGS with 365 days, or annual COGS with a single month's inventory, gives a number that means nothing.
- Chasing a lower DIO at any cost. Stock that runs out forfeits the margin on every order you could not fill. Balance days on hand against reorder points and lead times.
FAQ
DIO equals average inventory at cost divided by cost of goods sold for the period, multiplied by the number of days in that period. With A$180,000 of average inventory and A$270,000 of quarterly COGS, DIO is 0.667 × 90, or 60 days.
Lower usually means cash comes back faster and less stock risks going stale, but only if you are not running out. The right DIO covers your supplier lead time plus a safety buffer. Below that, you trade inventory days for lost sales.
None in practice. Days inventory outstanding, days sales of inventory and days in inventory are names for the same calculation: how many days, on average, inventory is held before it is sold. Check whether a source uses average or closing inventory.
Buy closer to demand with a working open-to-buy plan, reorder smaller quantities more often where supplier terms allow, clear slow SKUs before they become dead stock, and forecast by SKU rather than by category. Each shortens the time cash spends on shelves.
Updated September 2026