Calculate inventory days using either accounting values or physical stock. Use the financial method for DIO, or the operational method to estimate how many days your current units will cover.
Use this for financial reporting. Inventory and COGS must be measured at cost and refer to the same accounting period.
Results
CalculatedUse this for stock coverage. This method works in units and estimates how long current stock will last at the entered average daily demand.
Results
CalculatedFiguring out how long stock sits on your shelves before it moves takes two different forms, depending on whether you're reading the books or reading the boxes. This calculator handles both: Days Inventory Outstanding (DIO) for the financial view, and days of supply for the operational one — so you can approach the same question from whichever angle actually matters to you.
How to Calculate Days of Inventory
Start with average inventory:
Average Inventory = (Beginning Inventory + Ending Inventory) / 2
Then divide by cost of goods sold and multiply by the number of days in the period:
Days of Inventory = (Average Inventory / COGS) × Days in Period
For a full year, use 365 days. Nothing stops you from using a month, a quarter, or any custom stretch of time — just make sure the inventory figure and the COGS figure both cover that same window, or the number stops meaning anything.
Flip the ratio and you get inventory turnover:
Inventory Turnover = COGS / Average Inventory
Same underlying movement, two units of measurement. One counts in days, the other in cycles, and once you have either figure, the other is a quick conversion away.
Days of Inventory vs Days of Supply
DIO works well when you're reading financial statements, since both inventory and COGS sit on the books at cost.
Out on the warehouse floor, though, physical counts usually tell you more:
Days of Supply = Current Inventory / Average Daily Demand
Say you're holding 1,200 units and moving 40 a day on average — that's roughly 30 days of supply before you're out.
WareStat pushes this further and works out usable coverage before safety stock gets eaten into, expected demand across the supplier's lead time, a projected stockout date, and how much inventory you'd need to hit a target number of coverage days.
What Is a Good Days of Inventory Number?
There isn't one — not a universal one, anyway.
A lower DIO generally means stock is turning faster and less cash is parked on the shelf. Push it too low, though, and stockouts become a real risk. A high number points the other way: excess inventory, products that aren't selling, or purchasing that's running ahead of actual demand.
The comparison that actually tells you something is with your own history and with businesses that have similar products, margins, lead times, and demand patterns — not some generic industry average pulled from nowhere in particular.
Why Use COGS Instead of Revenue?
Because inventory sits on the books at cost, not at whatever it eventually sells for. Bring revenue into the calculation and you're blending inventory cost with selling price, a mix that turns misleading fast once products carry different margins.
Beginning and Ending Inventory or Average Inventory?
The beginning-and-ending approach is the easy one:
Average Inventory = (Beginning Inventory + Ending Inventory) / 2
But if your stock swings a lot — especially in seasonal businesses — that two-point average can miss what's actually happening in between. Several snapshots averaged together usually get closer to the truth.
Already know your average inventory figure? Select Average Inventory directly in the calculator and skip the two-point math entirely.
Related Inventory Calculations
Days of inventory earns its keep when it's paired with turnover, safety stock, and reorder point figures. On its own, none of them tell the whole story.
For a deeper look at turnover, GMROI, and historical trends, the WareStat Inventory Turnover Calculator picks up where this one leaves off. When the question is exactly when to place a replenishment order, that's what the WareStat Reorder Point Calculator is built for.
Frequently Asked Questions
Is Days of Inventory the same as DIO?
Close enough for most purposes. Days of Inventory, Days Inventory Outstanding, Days Sales of Inventory, and Inventory Days all describe essentially the same financial metric under different names.
Can I calculate inventory days using units instead of money?
Yes — divide current inventory units by average daily unit demand and you've got operational stock coverage instead of a financial figure.
Should safety stock be included?
If you want total stock coverage, yes. The calculator also breaks out the usable days you have before dipping into that safety-stock buffer.
Can I calculate DIO for a month or quarter?
Yes. Just enter the actual number of days in whichever period you're using, and keep COGS and inventory figures tied to that same window.