If you use the financial values of the inventory:
Inventory Turnover Ratio = Cost of Goods Sold / Average Inventory
The average inventory can be calculated as follows:
Average Inventory = (Beginning Inventory + End Inventory) /2
An Inventory Turnover Ratio is of course only valid if there were no changes in the costs of goods or a revaluation of the inventory during the reporting period.
Some ERP systems do not use the financial values, but the stock items as basis to calculate the Inventory Turnover. In that case it looks as follows:
Inventory Turnover = Number of Goods Sold / Average Number of Goods in Inventory
