DSI tells you how many days, on average, stock sits on the shelf before it sells. A DSI of 30 means the typical item waits a month between arriving and being sold. Lower means tight operations, money moving fast; higher means goods risk going stale while cash sits trapped in them. There is no universal good level: a supermarket turns stock in weeks while an industrial manufacturer can hold it past 150 days, so always compare within an industry.
The trend carries the real signal. A rising DSI while revenue is flat or falling is one of the clearest warnings in all of fundamental analysis: products are being made faster than they are selling, and stock is piling up. What usually follows is price cuts to clear the shelves, which crushes margins, or writing the stale stock off entirely. Catching that pattern early is exactly what this metric is for.
Two notes on our numbers. We leave DSI blank for businesses that barely hold inventory (under 1% of total assets), such as software, services, and banks, where DSO tells the working-capital story on its own. And since sales happen across a whole period while inventory is measured at a point in time, the inventory figure used is the average over the period, so the two line up.