FIFO is a cost layering concept under which the first goods purchased are assumed to be the first goods sold. The concept is used to devise the valuation of ending inventory, which in turn is used to calculate the cost of goods sold.

The FIFO concept (also known as first in, first out) is best shown with an example. ABC Company buys ten green widgets for $5 each in January, and an additional ten green widgets in February for $7 each. In March, it sells ten widgets. Based on the FIFO concept, the first ten units that ABC purchased should be charged to the cost of goods sold, on the theory that the first units into inventory should be the first ones removed from it. Thus, the cost of goods sold in March should be $50, while the value of the inventory at the end of March should be $70.

Even if some of the actual $7 green widgets were sold in March, the FIFO concept states that the cost of the earliest units should still be charged to the cost of goods sold first.

A company that uses FIFO will find that the costs it maintains in its records for its inventory will always be the most current costs, since the last items purchased are still assumed to be in stock. Conversely, the cost of the oldest items will be charged to the cost of goods sold. In a normal inflationary environment, this means that the cost of goods sold will be relatively low in comparison to current costs, which will increase the amount of taxable income; also, the inventory value reported on the balance sheet will approximately match current costs.

Alternative methods of accounting for inventory are the weighted average method, the last-in first-out method, and the specific identification method.

The FIFO concept also applies to the actual usage of inventory. When inventory items have a relatively short life span, it can be of considerable importance to structure the warehousing storage system so that the oldest items are presented to pickers first. Doing so reduces the risk of inventory spoilage.