This is a very simple calculation. Days to Sell Inventory(or Days in Inventory) = Average Inventory / Annual Cost of Goods Sold /365 Average Inventory = (Beginning Inventory + Ending Inventory) / 2 To calculate this ratio for a quarter instead of a year use the following variation: Days to Sell Inventory (or Days in Inventory) = Average Inventory / "Quarterly" Cost of Goods Sold /"90" Average Inventory = (Beginning Inventory + Ending Inventory) / 2