Calculating Days Inventory is a financial metric used to measure a company’s inventory efficiency. In essence, it shows the number of days it takes for a company to sell its current inventory. This means that a higher inventory turnover rate indicates more efficient usage of inventory and is usually considered favorable. To calculate Inventory Days you take the average value of your inventory for the period and divide it by the cost of goods sold for the same period. The result is the number of days it took for a company to sell its inventory. Keeping this number low is key in any successful business!