GAAP Inventory Valuation Methods: FIFO, LIFO, and Weighted Average

Under U.S. GAAP, you can value inventory using one of four methods: specific identification, first-in first-out (FIFO), last-in first-out (LIFO), or weighted average cost. The choice drives two numbers that lenders and investors read first: the inventory asset on your balance sheet and cost of goods sold on your income statement. From identical purchase records, these methods can produce meaningfully different results, so the decision is more consequential than most owners expect.

Here is how each one works, when it fits, and what the IRS and FASB require when you adopt or change it.

The Four Methods at a Glance

Specific identification attaches the actual cost paid to each individual item and carries that cost through to the sale. It’s appropriate, and generally required, when goods are unique and not interchangeable.

FIFO assumes the oldest costs leave inventory first and hit COGS. It typically mirrors the physical flow of goods and leaves the balance sheet carrying the most recent purchase prices.¹

LIFO assumes the newest costs are charged to COGS first. The physical flow rarely matches, but during inflation the method reduces taxable income, which is the point.¹

Weighted average cost blends all purchase costs into a single per-unit figure and applies it to both COGS and ending inventory. It suits large volumes of homogeneous, interchangeable goods.

How the Method Changes Your Numbers

Suppose you buy 100 units at $10 and then 100 units at $12, and you sell 150 units during the period. The three cost-flow methods produce different results from exactly the same transactions.

Under FIFO, COGS draws from the oldest purchases first: 100 units at $10 plus 50 units at $12, for $1,600. Ending inventory holds 50 units at the most recent $12 cost, or $600.

Under LIFO, COGS draws from the newest purchases first: 100 units at $12 plus 50 units at $10, for $1,700. Ending inventory holds the oldest 50 units at $10, or $500.

Under weighted average cost, the combined $2,200 outlay divided across 200 units gives $11 per unit. COGS on 150 units is $1,650, and ending inventory is $550.

The pattern during rising prices is consistent. FIFO gives the lowest COGS and highest ending inventory. LIFO gives the highest COGS and lowest ending inventory. Weighted average lands between the two on both measures. When prices fall, the relationships flip. When prices are stable, all three methods produce identical results.

Specific Identification

Specific identification works only for high-value, low-volume goods that aren’t interchangeable: custom machinery, fine art, real estate lots, luxury vehicles. If you buy a painting for $500,000, that exact cost stays with that painting until it sells.

The IRS expects you to use specific identification when you can match actual costs to individual items, and to fall back on FIFO, LIFO, or another method when items are interchangeable and commingled.¹ Accounting standards under ASC 330 take the same position: when similar goods are purchased at different times and prices, their identity is lost between acquisition and sale.

The method carries a manipulation risk auditors watch for. Because management chooses which specific unit to record as sold, they can steer reported income by picking a high-cost item (reducing profit) or a low-cost item (inflating it). That discretion is precisely why GAAP steers interchangeable-goods businesses toward a cost flow assumption instead.

FIFO

FIFO is the most widely used inventory method globally. It’s one of only two cost formulas permitted under International Financial Reporting Standards, with weighted average cost being the other.²

During inflation, FIFO matches older, cheaper costs against current revenue, which produces the lowest COGS and the highest reported income of the three cost-flow methods. The balance sheet carries recent purchase prices, giving a current picture of asset value. During deflation, the effect reverses.

LIFO

LIFO exists primarily as a tax strategy. By charging the newest, highest costs against revenue during inflation, it produces the highest COGS and the lowest taxable income of any permitted method. The physical flow of goods is beside the point.¹

Three features of LIFO reshape the decision and deserve attention before you elect it.

The LIFO Conformity Rule

If you elect LIFO on your federal return, you must also use LIFO in any financial statements you issue to shareholders, creditors, or other stakeholders. This is the LIFO conformity rule in Section 472 of the Internal Revenue Code.³ You cannot report lower income to the IRS using LIFO while showing investors a rosier picture using FIFO.

If you violate the rule by using a non-LIFO method in any report to shareholders or for credit purposes, the IRS can require you to abandon LIFO entirely for tax purposes.⁴ The conformity requirement extends to all members of a group of financially related corporations, not just the entity that made the election.³

The LIFO Reserve and Liquidation Risk

Over years of rising prices, LIFO companies accumulate old inventory layers carried at costs well below current market value. The gap between inventory reported under LIFO and what it would be under FIFO is the LIFO reserve, which companies typically disclose so investors can compare them to FIFO competitors.

Trouble surfaces during a LIFO liquidation, when you sell more inventory than you replace in a period. Those old, low-cost layers finally hit COGS, income spikes, and the tax bill jumps with it. Years of deferred taxes come due at once, and supply disruptions or deliberate inventory drawdowns can trigger it unexpectedly.

Not Available Internationally

LIFO is a U.S.-only method. IFRS limits companies to FIFO and weighted average cost, with specific identification reserved for non-interchangeable items.² U.S. companies on LIFO that report to international stakeholders typically provide supplemental disclosures reconciling to a FIFO or average-cost basis.

Weighted Average Cost

Weighted average cost suits bulk chemicals, agricultural commodities, raw materials, and other goods where tracking individual purchase layers would be pointless. Its defining trait is smoothing: price swings are averaged into a single per-unit figure rather than amplified at either end.

How you compute the average depends on your inventory system. In a periodic system, you calculate one weighted average at the end of the period using all purchases made during it. In a perpetual system, you recalculate a moving average after every new purchase, so the per-unit cost shifts throughout the period. The two approaches produce slightly different COGS and ending inventory figures because each sale in a perpetual system draws from the running average in effect at that moment rather than a single end-of-period figure.

The Lower of Cost or NRV Ceiling

After you’ve assigned costs using one of the methods above, GAAP imposes a ceiling test. Inventory cannot sit on the balance sheet at more than you can realistically get for it.

Companies using FIFO or weighted average compare cost to net realizable value: estimated selling price minus remaining costs to complete, sell, and ship. If NRV is lower than cost, you write the inventory down and record the difference as a current-period expense.⁵ Common triggers are physical damage, obsolescence, and sharp drops in market prices. A manufacturer holding laptops that cost $800 but can now sell for $750 after a competitor’s launch writes the units down to $750.

Companies using LIFO or the retail inventory method apply the older “lower of cost or market” test instead. “Market” generally means current replacement cost, capped by NRV on top and NRV minus a normal profit margin on the bottom. ASU 2015-11 simplified the rule for other companies but left LIFO and retail-method users on the older framework.⁵

Once you write inventory down, the reduced amount becomes the new cost going forward. Even if market conditions recover next quarter, U.S. GAAP does not permit reversing the write-down. IFRS allows reversals up to original cost; U.S. rules do not.

Electing or Changing Your Method

Your inventory method is both an accounting policy and a tax election, and the IRS controls when you can start or switch.

Electing LIFO

You elect LIFO by filing Form 970 with the tax return for the first year you want to use it.⁶ Once adopted, LIFO continues in all later years unless the IRS approves a change or revokes the election for a conformity violation.³ Your financial statements must switch to LIFO the same year.⁷

Switching Methods

To change methods after initial adoption, you file Form 3115, Application for Change in Accounting Method. Many inventory method changes qualify for automatic consent: the IRS grants approval as long as you follow the published procedures, and no user fee applies. Non-automatic changes require a user fee and a formal IRS ruling.⁸

Any method change triggers a Section 481(a) adjustment, which prevents income from being counted twice or skipped during the transition. A net increase in taxable income is generally spread over four tax years. A net decrease is taken entirely in the year of change.⁹ Build the adjustment into cash flow projections before you decide a switch makes sense.

Small Business Exemption

Not every business needs formal inventory accounting. Under Section 471(c), taxpayers that meet the gross receipts test in Section 448(c) can treat inventory as non-incidental materials and supplies or follow the method reflected in their financial statements. Qualifying small businesses can effectively skip the cost flow assumption and deduct inventory costs when goods are sold or used.¹⁰

What You Have to Disclose

GAAP requires the inventory accounting policy to appear in footnotes. At minimum, disclose which cost flow method you use and apply it consistently from period to period. The method must conform to generally accepted accounting principles for similar businesses and clearly reflect income.¹

The balance sheet figure is typically broken out by category: raw materials, work-in-process, and finished goods. Substantial or unusual losses from lower-of-cost-or-NRV write-downs should be identified separately rather than buried in COGS.⁵ LIFO companies should also disclose the LIFO reserve so investors can restate inventory and COGS on a FIFO-equivalent basis.

When you change methods, disclose the nature and reason for the change in the first reporting period after adoption.⁵ Analysts pay particular attention to switches away from LIFO, because the Section 481(a) adjustment can move reported earnings significantly in the transition year.

1
Internal Revenue Service. Publication 538 – Accounting Periods and Methods
2
IFRS Foundation. IAS 2 Inventories
3
Office of the Law Revision Counsel. 26 USC 472 – Last-in, First-out Inventories
4
Internal Revenue Service. LIFO Conformity
5
Financial Accounting Standards Board. ASU 2015-11 – Inventory (Topic 330)
6
Internal Revenue Service. About Form 970 – Application to Use LIFO Inventory Method
7
Internal Revenue Service. Adopting LIFO
8
Internal Revenue Service. Instructions for Form 3115
9
Office of the Law Revision Counsel. 26 USC 481 – Adjustments Required by Changes in Method of Accounting
10
Office of the Law Revision Counsel. 26 USC 471 – General Rule for Inventories