Return on Inventory Calculator determines if a lower price per unit for a larger order is worth the extra inventory investment. To use Return on Inventory Calculator, you key in: your selling price, unit cost, the quantity you wish to buy or make of this item, and how many you expect to sell per month on average in the coming year. It then calculates your: yearly sales, gross profit, gross profit %, month's supply quantity, average inventory, and return on inventory.