Cost Flow Assumption

A cost flow assumption is an accounting rule that determines the order in which costs are treated as leaving a balance sheet account, most commonly inventory, as units are sold or otherwise used. The key insight behind the term is that this assumed order does not have to match the actual, physical movement of the goods themselves; a grocery store physically sells its oldest milk first for freshness reasons, but could in principle still use a cost flow assumption that treats the most recently purchased costs as sold first, since the accounting assumption and the physical flow of goods are two separate things.

Under first-in, first-out (FIFO), the oldest costs in inventory are assumed to be the first ones removed when a sale occurs, leaving the most recently incurred costs in ending inventory. Under last-in, first-out (LIFO), the most recently incurred costs are assumed to be removed first, leaving the oldest costs in ending inventory. The weighted-average cost flow assumption blends all available costs together into a single average cost per unit, applying that average both to units sold and to units remaining in inventory, rather than treating any particular batch of costs as leaving before another.

During a period of rising prices, FIFO tends to produce a lower cost of goods sold and therefore higher reported profit, since the oldest, cheaper costs are the ones matched against current sales, while LIFO produces a higher cost of goods sold and lower reported profit, since the newest, more expensive costs are matched against sales instead. The same relationship affects the balance sheet in reverse: FIFO leaves the newest, higher costs in ending inventory, while LIFO leaves the oldest, often significantly understated, costs in ending inventory. Choosing a cost flow assumption is therefore not a purely mechanical bookkeeping decision; it has a direct, sometimes substantial effect on a company's reported profit, tax liability, and the apparent value of its inventory.

While inventory is the most common context, the same concept applies to other assets that are acquired at different costs over time and later sold off in pieces, including investment securities and treasury stock repurchased by a corporation at different prices on different dates. Whenever an asset is accumulated through multiple purchases at varying costs and then partially disposed of, some cost flow assumption is needed to determine which specific costs are treated as having left the books.

Accounting standards require a company to disclose which cost flow assumption it uses and to apply that method consistently from period to period, under the broader guideline of consistency, so that readers of the financial statements can meaningfully compare results across periods. In the United States, a company that chooses LIFO for its income tax return is also generally required, under the LIFO conformity rule, to use LIFO for its externally reported financial statements as well, preventing a company from reporting the more favorable, typically higher-profit FIFO figures to investors while simultaneously claiming the tax benefits of LIFO on its tax return.