Days Inventory Outstanding (DIO) shows how long stock remains on hand before it is sold. It is a key measure of supply chain efficiency, cash flow health, and inventory performance. In this guide, we explain how to calculate DIO, what high or low results mean, and how to use it to strengthen forecasting and replenishment. You will also learn how different industries benchmark DIO and which practical steps reduce it. With the right tools and planning, you can improve stock rotation and free working capital.