LIFO vs FIFO: Inventory Accounting Explained

For a company that sells physical goods, inventory is one of the largest items on the balance sheet and the single biggest driver of cost of goods sold. So when a candidate is asked "what is the difference between LIFO and FIFO, and what happens to the financials in an inflationary environment?", the interviewer is testing something more important than a definition. They want to see whether you understand that an accounting choice with no effect on the actual business can move reported earnings, taxes, and the balance sheet at the same time, and whether you can trace that effect through all three statements.

This post walks through what each method does, a clean numerical example, the LIFO reserve that lets you convert between them, the tax and cash-flow consequences, the trap of LIFO liquidation, and why US companies can choose LIFO while companies reporting under IFRS cannot. By the end you should be able to answer the interview question fluently and adjust a real company's financials for the method it uses.

The key idea to anchor on first: LIFO and FIFO are about the flow of costs, not the flow of physical goods. A grocery store still sells its oldest milk first regardless of which method it uses for accounting. The method only decides which purchase cost gets matched against revenue when a unit is sold.

Before the mechanics, here is the comparison every interviewer wants you to be able to reproduce. Assume a period of rising prices, which is the normal and most-tested case.

The whole table flips if prices are falling, but inflation is the default scenario in interviews and the reason LIFO exists at all. Notice the central tension: LIFO gives you a more realistic income statement (recent costs matched against recent revenue) at the expense of a less realistic balance sheet (old costs stuck in ending inventory), while FIFO does the reverse.

Both methods solve the same problem. A company buys identical units at different prices over time, then sells some of them. Accounting needs a rule to decide which cost left the balance sheet and became an expense. FIFO and LIFO are two such rules.

FIFO: oldest costs to the income statement

FIFO (First In, First Out)

An inventory cost-flow method that assigns the earliest (oldest) costs to cost of goods sold and leaves the most recent costs in ending inventory. In a period of rising prices, FIFO produces lower COGS, higher net income, and an ending inventory balance that closely reflects current replacement cost.

Under FIFO, the income statement carries stale costs into COGS while the balance sheet holds fresh ones. When prices rise, that mismatch inflates reported profit because you are selling at today's prices but expensing yesterday's lower costs. The gap between the two is sometimes called inventory profit or phantom profit, because it reflects price changes rather than genuine operating performance.

LIFO: newest costs to the income statement

LIFO (Last In, First Out)

An inventory cost-flow method that assigns the most recent (newest) costs to cost of goods sold and leaves the oldest costs in ending inventory. In a period of rising prices, LIFO produces higher COGS, lower net income, lower taxable income, and an ending inventory balance that can be badly understated relative to current cost.

LIFO does the opposite. By pushing the newest, highest costs into COGS, it matches current costs against current revenue, which many argue gives a truer picture of operating margins during inflation. The cost of that cleaner income statement is an increasingly outdated inventory figure on the balance sheet, because the old cost layers can sit there for years or even decades.

Numbers make this concrete. Imagine a distributor that buys a single product in two batches as prices rise during the year, then sells half of what it bought.

Both methods agree on revenue ($2,000) and on total inventory purchased ($2,400). They disagree only on which cost layer left when those 100 units were sold.

Running the numbers

Under FIFO, the units sold are assumed to be the oldest, so COGS uses the January batch:

Ending inventory is the newer June batch, $1,400. Gross profit is $2,000 minus $1,000, or $1,000.

Under LIFO, the units sold are assumed to be the newest, so COGS uses the June batch:

Ending inventory is the older January batch, $1,000. Gross profit is $2,000 minus $1,400, or $600.

Where the $400 difference comes from

The profit difference is not random. It equals the price increase ($4 per unit) times the units sold (100), which is $400. That is the inflation embedded in the period, and the two methods simply allocate it differently: FIFO reports it as profit now, LIFO defers it inside the old inventory layer.

Because LIFO can distort the balance sheet, US GAAP requires any company using LIFO to disclose the LIFO reserve in its filings, almost always in the inventory footnote. This single number lets an analyst translate a LIFO company onto a FIFO basis so it can be compared with peers.

LIFO reserve

The difference between a company's inventory valued under FIFO and its inventory valued under LIFO. It represents the cumulative amount by which LIFO has reduced reported inventory (and, through higher cumulative COGS, deferred taxable income) since the company adopted the method. It is disclosed in the financial statement footnotes.

The conversion formula

Formally, the relationship is simple:

In the worked example, the LIFO reserve is $1,400 minus $1,000, or $400. To restate a LIFO company to FIFO, you add the reserve back to inventory, and you adjust COGS by the change in the reserve over the year:

A growing reserve means LIFO is expensing more than FIFO would in that period, so to get back to FIFO COGS you subtract the increase. This is the exact adjustment analysts make so that a LIFO oil major and a FIFO competitor can be compared on the same footing, which matters when you are building a three-statement model or linking the three statements for two companies side by side.

A real example: ExxonMobil

Real filings show how large the reserve can get. In its fiscal-year 2024 Form 10-K, ExxonMobil reported inventories of $23,524 million on a LIFO basis and disclosed that the figure would have been roughly $10 billion higher under FIFO (SEC Form 10-K, filed February 2025). That ~$10 billion gap is the accumulated effect of decades of rising energy and input prices sitting in old LIFO layers. Anyone valuing the company on book value or inventory turnover without adjusting for that reserve would be working with a badly understated asset base.

If LIFO makes earnings look worse, why would any company choose it? The answer is cash. In the United States, the LIFO conformity rule in the tax code says that a company using LIFO for its tax return must also use LIFO in its financial statements. Companies accept lower book earnings precisely because LIFO lowers taxable income when prices rise, and lower taxable income means a smaller cash tax bill.

The tax saving in numbers

In the worked example, LIFO reported $400 less pretax profit. At a 25% tax rate, that is $100 of taxes deferred in a single year on a tiny amount of inventory. Scale that to a company like ExxonMobil with a ~$10 billion reserve and the deferred tax is measured in billions. This is why LIFO is concentrated in industries with large, slow-turning inventories and persistent input-cost inflation: oil and gas, industrials, automotive parts, and some large retailers.

The valuation angle

This also explains a subtle point about valuation. Because LIFO usually produces a lower, more conservative measure of operating profit during inflation, some analysts view LIFO earnings as higher quality. But it depresses metrics like EBITDA relative to a FIFO peer, so any clean comparison or normalized EBITDA analysis has to put both companies on the same inventory basis first.

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LIFO has a failure mode that interviewers love to test because it catches people who memorized the rule without understanding it. It happens when a LIFO company sells more units than it buys in a period, so it has to dip into old, cheap cost layers to fill the sales.

LIFO liquidation

A situation where a company using LIFO sells more inventory than it purchases in a period, forcing it to expense old, low-cost inventory layers into COGS. This temporarily reduces COGS, inflates gross profit and net income, and raises the tax bill, the opposite of LIFO's normal effect.

The result is perverse. A company that has been quietly deferring taxes for years suddenly reports a profit spike, not because the business improved, but because decades-old $10 units are flowing into COGS against today's $20 selling prices. Earnings look great and then the cash tax bill arrives. This is why a sharp jump in a LIFO company's margins deserves scrutiny: it can signal inventory drawdown rather than genuine operating strength.

One of the cleanest ways to show depth is to explain that LIFO is essentially an American phenomenon. US GAAP permits FIFO, LIFO, and weighted-average cost. IFRS, the standard used across Europe, the UK, and most of the world, permits FIFO and weighted-average but prohibits LIFO under IAS 2.

Why the IASB rejected it

The reasoning is about the balance sheet. The IASB concluded that LIFO does not faithfully represent inventory flows and tends to leave inventory carried at old, irrelevant costs that no longer reflect economic reality (KPMG, inventory accounting under IFRS vs US GAAP). IFRS leans toward a balance sheet stated at amounts closer to current value, and LIFO's stranded cost layers run directly against that goal. There is also a widely held view that LIFO is used primarily to reduce taxes rather than to improve reporting, which sits poorly with a principles-based framework.

What it means for comparison

For an interview, the practical implication matters most: a European or Asian company you are comparing will never be on LIFO, so a US LIFO company must be restated to FIFO before any cross-border comparison of margins, inventory turnover, or valuation multiples. This is a real issue in cross-border M&A, where a US target on LIFO and an acquirer reporting under IFRS have to be reconciled.

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The reason this topic is a favorite is that it touches all three statements at once, which is exactly the skill a three-statement question tests. Walk it through for an inflationary period, switching a company from FIFO to LIFO:

The elegant part of the answer is the apparent paradox: lower net income but higher cash. Resolving that paradox out loud, the difference is the deferred cash taxes, is what separates a candidate who understands the mechanics from one who memorized a table. The same logic appears in the canonical interview drills covered in walking through the three statements.

A few errors come up again and again, both in interviews and in real analysis:

LIFO versus FIFO is a perfect interview topic because it looks like a narrow accounting rule and turns out to touch earnings, taxes, cash flow, the balance sheet, and cross-border comparability all at once. The candidate who can explain that the choice is a cost-flow assumption, trace its effects through the three statements, resolve the lower-income-but-higher-cash paradox, and adjust a real company using its LIFO reserve is demonstrating exactly the integrated thinking that banking technicals are designed to surface.

When you study this, do not just memorize the direction of the arrows in the comparison table. Practice the worked example until you can generate the numbers yourself, then practice restating a real 10-K from LIFO to FIFO. That fluency, the ability to move between an accounting choice and its full financial consequences, is what turns a memorized definition into a confident, complete answer.