A business that manufactures products to be sold, or purchases products for resale, must value its product inventory at the beginning and the end of each tax year to determine the cost of goods sold (COGS) during the year. The business determines its gross profits by deducting COGS from its gross receipts for the year. The business then deducts its other business expenses from gross profits, to determine its net (taxable) income for the year.

Certain expenses are included in COGS. Expenses that are included in COGS cannot be deducted again as a business expense. COGS expenses include:

• The cost of products or raw materials, including freight or shipping charges;
• The cost of storing products the business sells;
• Direct labor costs for workers who produce the products; and
• Factory overhead expenses.

Purchased inventory

If the business purchases its inventory for resale, its inventory costs are the invoice price plus transportation and other necessary expenses, less discounts.  Discounts that must be deducted from the costs of purchased inventory include trade discounts, manufacturer's rebates, and cash discounts.

Trade discounts are a reduction in the price of goods that a manufacturer or wholesaler provides to a retailer.  It includes a discount that is always allowed, regardless of the time of payment. A manufacturer's rebate is based on the dealer's purchases during the year. A cash discount is a reduction in the invoice price that the seller provides if the dealer pays immediately or within a specified time. The cash discount may reduce COGS, or it may be treated separately as gross income. Certain excise tax reimbursements may reduce the value of ending inventory and therefore reduce COGS.   

Methods of accounting

It is usually impractical to associate items of intermingled or fungible inventory with specific invoices and costs. Instead, taxpayers use certain assumptions or methods of accounting to identify the goods on hand and their costs. The traditional assumptions include FIFO (first-in, first-out) and LIFO (last-in, first-out). In some cases, specific identification is used. The courts have approved the average cost method, although the IRS disagrees with its use. The IRS will permit taxpayers to use other inventory cost assumptions, such as the rolling-average method, if they are reasonable for the taxpayer's trade or business and clearly reflect income.

Under the FIFO, the taxpayer is presumed to sell the oldest goods in inventory and to retain the most-recently produced or purchased items. If production (inventory) costs are rising, the use of FIFO reduces COGS and increases the taxpayer's income. Under LIFO, the taxpayer is presumed to sell the most recently obtained goods and to retain the oldest goods in inventory. Assuming that inventory costs are rising, the LIFO method will increase COGS and decrease the taxpayer's income. Under the average cost method, all units purchased during the year are averaged with the cost of beginning inventory, to determine an average cost.

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