Days Inventory Outstanding (DIO): Formula and Meaning | Inventory Hero
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Days Inventory Outstanding (DIO): Formula and Meaning
Days inventory outstanding measures how many days it takes to sell your stock, at cost. The formula, a worked example, and how DIO drives your cash cycle.
Days inventory outstanding (DIO), also called days sales of inventory, is the average number of days it takes to sell through your inventory, measured at cost. It answers how long your money sits in stock before that stock becomes a sale. A lower DIO means you turn inventory into cash faster, which frees working capital, while a high DIO means cash is stuck on the shelf longer.
How do you calculate days inventory outstanding?
DIO equals average inventory divided by cost of goods sold, multiplied by the number of days in the period. For example, average inventory of 50,000 dollars, COGS of 300,000 dollars over a year, gives (50,000 / 300,000) x 365, which is about 61 days. Use average inventory (beginning plus ending, divided by two) and match the COGS period to the days you multiply by.
T. Brian Jones is co-founder and CTO of Inventory Hero. He leads the engineering behind its Amazon data pipeline, demand forecasting, and the AI platform that lets sellers talk to their live inventory, sales, and supplier data in plain language.
What is the difference between DIO and days of supply?
They measure the same underlying idea, how long inventory lasts, from different angles. DIO is the accounting view, based on inventory value and COGS over a past period. Days of supply is the operational view, based on units on hand divided by daily sales velocity, and is forward-looking for reorder decisions. DIO tells you how efficiently capital moved through inventory; days of supply tells you when to reorder.
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Days inventory outstanding (DIO) measures the average number of days it takes to sell through your inventory, valued at cost. The short version: it is average inventory divided by COGS, times the days in the period, a lower number means you turn stock into cash faster, and it is the inventory leg of your cash conversion cycle. Below is the formula, a worked example, and how DIO fits your cash picture.
Days inventory outstanding is calculated over a period:
DIO = (average inventory / COGS) x days in the period
Average inventory is beginning plus ending inventory divided by two, at cost.
COGS is your cost of goods sold over the same period.
Days in the period is 365 for a year, 90 for a quarter, and so on.
The ratio of average inventory to COGS is the fraction of a period's cost sitting in stock at any time; multiplying by the days converts that fraction into a number of days. It is the same information as inventory turnover (which the turnover calculator works out for you), expressed as days rather than turns.
Say over a year your average inventory is 50,000 dollars (at cost) and your COGS is 300,000 dollars:
DIO = (50,000 / 300,000) x 365 = 60.8 days
So on average it takes about 61 days to sell through your inventory. If a competitor runs the same category at 40 days, they are cycling their cash through inventory roughly half again as fast as you, which means the same cash supports more sales. That is the practical stakes of DIO: it is a measure of how hard your inventory capital is working.
DIO is not just an efficiency stat; it is one of the three legs of your cash conversion cycle:
Cash conversion cycle = DIO + days sales outstanding - days payable outstanding
DIO is how long cash sits in inventory.
Days sales outstanding is how long until customers pay (short for Amazon, which pays on a schedule).
Days payable outstanding is how long you take to pay suppliers.
Lowering DIO directly shortens your cash cycle, which means less working capital tied up and more available to fund growth. For an FBA business where cash is usually the binding constraint, DIO is a lever worth watching.
DIO and days of supply are two views of the same idea, and confusing them is common:
DIO is the accounting, cost-based, backward-looking view: how efficiently capital moved through inventory over a past period, from your financials.
Days of supply is the operational, units-and-velocity, forward-looking view: how many days your current stock will last at your recent sales pace, used to time reorders.
Use DIO to judge capital efficiency and feed your cash cycle; use days of supply to decide when to reorder a specific SKU. They agree in spirit but answer different questions.
Lower is usually better, but too low can mean you are running so lean you risk stockouts, so DIO is best read against your service level.
Compare to yourself over time and to your category, since a "good" DIO varies enormously between fast-moving consumables and slow, high-margin goods.
Watch the trend. A rising DIO means inventory is building faster than it sells, an early warning of overstock and a cash-cycle problem forming.
For an FBA anchor: a SKU's DIO should not run much beyond its full replenishment cycle, meaning your supplier lead time plus the safety days you want to hold. A DIO consistently longer than that lead time is a signal you are overbuying that SKU. That is a rule of thumb, not an Amazon-published figure, and a healthy DIO varies sharply between fast-moving consumables and slow premium goods, so track the trend per SKU rather than chasing an industry number. DIO is a diagnostic: the number itself matters less than what a change in it is telling you about how your inventory and cash are moving.
If your DIO is higher than you want, the levers are the same ones that improve turnover:
Clear the slow movers. The SKUs dragging your average are the aged, low-turn products; reducing them pulls average inventory down and DIO with it.
Buy tighter. Ordering closer to demand, rather than overbuying to hit a supplier minimum, keeps average inventory lower for the same sales.
Improve forecasting. Better demand forecasts mean you hold less safety stock for the same service level, lowering the inventory side of the ratio.
Speed up the slow segment. Bundles, promotions, or repricing on stagnant stock lift its sell-through, which raises COGS flow-through and cuts days.
One caution: do not chase a low DIO by starving your winners of stock. A DIO that drops because you are stocking out is a false win, since the lost sales cost more than the freed cash. The goal is a low DIO at a healthy service level, which means cutting the dead weight, not the buffer your fast sellers need.
Days inventory outstanding is the average days it takes to sell your stock at cost, calculated as average inventory over COGS times the period days. A lower DIO means faster cash cycling and less capital frozen in inventory, which is why it is the inventory leg of your cash conversion cycle. Read it against your service level and its trend, and use it alongside the operational days of supply view. For the wider cash picture, see cash conversion cycle and inventory turnover; for the FBA planning system that acts on this number, restock planning.