Inventory Cost Flow Assumptions: 4 Methods Compared

Inventory cost flow assumptions are the rules a company uses to decide which purchase costs go to cost of goods sold (COGS) and which stay in ending inventory. The four main methods are specific identification, first-in, first-out (FIFO), last-in, first-out (LIFO), and weighted average. U.S. GAAP allows all four. IFRS does not allow LIFO.

Why do you need a rule at all? Start with the goods themselves. A dealer can trace the cost of a particular concert piano to its serial number. A distributor selling thousands of identical cables needs a consistent rule for assigning the cost of each shipment to sales. That rule can change reported profit even when the same customers buy the same goods at the same prices.

You’ll see this topic on the Financial Accounting and Reporting (FAR) section of the CPA exam, in the external financial reporting part of CMA Part 1, and in EA Part 2, where the tax rules for LIFO come into play.

Prefer to watch first? Professor Farhat covers FIFO at 2:06, weighted average at 2:53, periodic versus perpetual systems at 3:10, and a full worked example at 4:40. Listen for the difference between choosing which box to ship and choosing which cost to record. Watch Professor Farhat’s explanation on YouTube.

What do inventory cost flow assumptions measure?

Suppose a stockroom has ten identical cables. A count establishes that ten cables exist. Counting tells you how many cables you have. It doesn’t tell you what they cost. If all ten cost $8 each, the answer is $80 under every method. If some cost $8 and others cost $11, the accountant needs to know which costs are still on hand.

Keep two separate records in mind. Physical flow describes which units employees move. Cost flow describes which invoice costs enter expense. A warehouse can rotate older stock forward to limit damage while its accounting system assigns an average cost to every unit sold.

What are the four inventory cost flow methods?

What does ‘under the chosen system’ mean in the LIFO row? It depends on how the company tracks inventory. Under a periodic system, costs are assigned at the end of the period, so LIFO uses the latest costs of the whole period. Under a perpetual system, costs are assigned at each sale, so LIFO uses the latest costs on hand on the sale date. Weighted average works the same way: one average for the whole period under periodic, or a new moving average after each purchase under perpetual.

For interchangeable goods, FIFO, LIFO, and weighted average each give a repeatable answer without tracing a particular carton. Specific identification uses evidence about the particular item delivered. For a custom instrument, that might be a serial number on both the purchase invoice and the sales order.

Identification has limits. If identical goods carry different costs, choosing which physical unit to deliver can also influence this period’s margin. Even a reliable serial number may leave some judgment about allocating a shared freight bill across a shipment.

Worked example using the same purchases

Maple Sound Supply, a hypothetical retailer, begins with no inventory. It buys 12 microphone stands at $24 each and then 18 at $34 each. After both deliveries, it sells 16 stands. That leaves 14 on hand. The two invoices total $900, calculated as (12 × $24) + (18 × $34).

For the specific identification row only, assume that shipment records trace seven of the sold stands to the first delivery and nine to the second. This extra fact is essential. A unit count alone cannot reveal the cost of those particular stands.

Read across the FIFO row. Once Maple assigns all 12 older costs to sales, it still needs four newer costs to price the 16-unit sale. The 14 remaining costs are all $34. Check the dollars as well as the units. Under each method, COGS plus ending inventory equals $900.

One more check: Maple received both deliveries before it made the sale, so a periodic system and a perpetual system give the same answers here. If sales happen between purchases, LIFO and average cost can give different results under the two systems. FIFO gives the same answer either way. See how the timing works in our periodic vs perpetual inventory costing walkthrough.

How do the methods change profit when prices rise?

Maple’s unit cost climbed from $24 to $34, so its numbers show the pattern exam questions test most often. To see the effect on profit, assume Maple sold the 16 stands for $50 each, or $800 in total.

When purchase costs rise, FIFO generally gives the lowest COGS, the highest gross profit, and the highest ending inventory, all else equal. LIFO gives the highest COGS, the lowest gross profit, and the lowest ending inventory. When prices fall, the pattern flips.

The gap between FIFO and LIFO inventory is called the LIFO reserve. For Maple, that’s $476 minus $356, or $120. SEC rules require public companies that use LIFO to disclose, if material, how much higher their inventory would be at replacement or current cost. Many companies present this as the LIFO reserve, which helps readers compare them with companies that use FIFO.

Higher COGS can reduce taxable income when the same inventory treatment applies for tax purposes, all else equal. This potential tax benefit is a major reason a business might choose LIFO when costs are rising. There’s a catch, though. Under the LIFO conformity rule in Internal Revenue Code section 472(c), a company that uses LIFO on its federal tax return must also use LIFO in the financial reports it gives owners and lenders. A business elects LIFO for tax purposes by filing IRS Form 970. For the full trade-off, see our FIFO vs LIFO profit and tax walkthrough.

Which inventory method fits the goods?

First ask whether distinguishing one unit from another is practical and meaningful. A marine dealer selling individually outfitted boats can document actual costs. A business that mixes identical fasteners in one bin may find a pooled average easier to apply and easier for another accountant to reproduce.

Next identify the reporting framework. U.S. GAAP permits specific identification, FIFO, LIFO, and average cost, as well as the retail inventory method. ASC 330-10-30-9 says the main objective is to choose the cost flow assumption that most clearly reflects periodic income.

IFRS does not permit LIFO. IAS 2 prescribes specific identification for items that are not ordinarily interchangeable and FIFO or weighted average for ordinarily interchangeable items. The same cost formula must be used for inventories of a similar nature and use. See the IFRS Foundation’s summary of IAS 2.

Source note: Paragraphs 5–8 of the agenda prioritization section in the December 2024 FASB staff meeting handout summarize the U.S. methods; the handout is discussion material, not an official FASB position. The FASB’s fourth-quarter 2024 Chair Report records the Board’s decision not to add a LIFO project to its agenda.

Finally, consider what readers need to understand. FIFO generally leaves recent purchase costs on the balance sheet. LIFO can charge more recent costs against current sales. If purchases and sales alternate, you also need to know when the accounting system prices each sale.

What happens if inventory loses value?

Cost flow assumptions allocate costs between COGS and ending inventory. A second rule decides whether that cost can stay on the balance sheet.

Under U.S. GAAP, inventory measured with FIFO, average cost, or specific identification is reported at the lower of cost and net realizable value (NRV). Net realizable value is the estimated selling price minus the reasonably predictable costs to complete, sell, and ship the goods. Inventory measured with LIFO or the retail inventory method still uses the lower of cost or market test. See FASB’s inventory measurement update (ASU 2015-11) for this distinction. Under IFRS, IAS 2 uses lower of cost and NRV for all inventory.

Here’s a quick example. Say Maple’s 14 FIFO stands carry a cost of $476, but a new competitor pushes prices down so each stand can now sell for only $31, with $2 of selling cost per stand. NRV is 14 × ($31 − $2) = $406. Maple writes the inventory down by $70, which is $476 minus $406.

Under U.S. GAAP, that write-down can’t be reversed later. IFRS allows a reversal if the value recovers, but never above original cost.

Practice questions

1. Which cost follows a custom instrument?

A dealer sells serial number P42, purchased for $6,300. A similar instrument cost $6,750. Answer: Under specific identification, COGS is $6,300 because the invoice for P42 identifies the cost of the instrument delivered.

2. Why would averaging $24 and $34 give the wrong answer for Maple?

Answer: The simple average of $29 gives equal weight to the two invoices. Maple bought more units at $34. Divide $900 by 30 units to get the weighted average of $30 per stand.

3. Could Maple report $544 of COGS and $476 of ending inventory?

Answer: Those amounts would allocate $1,020 from only $900 of purchases. They combine LIFO expense with FIFO inventory. Use one method for both sides of the allocation.

Frequently asked questions

Does FIFO require warehouse employees to ship the oldest unit?

FIFO assigns the oldest available cost to sales. Employees may arrange shipments by expiration date, customer specification, or location.

Can specific identification produce a figure between FIFO and LIFO?

Yes. Maple’s $474 expense falls between FIFO’s $424 and LIFO’s $544. Its position depends on the actual units sold. Under a periodic count, when unit costs only rise or only fall during the period, as in Maple’s case, specific identification lands between FIFO and LIFO or matches one of them. When costs rise and then fall, it can land outside that range. So there is no fixed ranking for specific identification.

Where can I find the method a public company uses?

Look in the inventory or significant accounting policies note to its financial statements. Check whether the company describes different product groups before applying a method to the entire reported balance.

Which inventory method gives the highest net income when prices rise?

FIFO generally gives the highest net income when purchase costs rise, all else equal. It sends the oldest, cheapest costs to COGS. In Maple’s example, FIFO COGS is $424, compared with $544 under LIFO. When purchase costs fall, LIFO generally gives the highest net income, all else equal.

Is LIFO allowed under IFRS?

No. IAS 2 allows FIFO or weighted average for interchangeable items and specific identification for items that aren’t interchangeable. LIFO remains allowed under U.S. GAAP.

What is the LIFO conformity rule?

If a company uses LIFO on its U.S. federal tax return, Internal Revenue Code section 472(c) requires it to use LIFO in its reports to owners and lenders too. A business elects LIFO by filing IRS Form 970.

Do periodic and perpetual systems give different answers?

FIFO gives the same answer under both. LIFO and average cost can differ when sales happen between purchases, because a perpetual system assigns costs on each sale date. In Maple’s example, both deliveries arrived before the sale, so the answers match.

Key Takeaways

• Inventory cost flow assumptions split the cost of goods available for sale between COGS and ending inventory. They don’t have to match how goods physically move.
• The four methods are specific identification, FIFO, LIFO, and weighted average. U.S. GAAP allows all four. IFRS doesn’t allow LIFO.
• Under every method, COGS plus ending inventory equals the cost of goods available for sale. For Maple, that’s $900.
• When purchase costs rise, FIFO generally reports the highest profit and ending inventory, and LIFO the lowest, all else equal.
• Use a weighted average (total cost ÷ total units), not a simple average of unit prices.
• A company that uses LIFO for U.S. taxes must also use it for financial reporting.

About the Author

Mansour Farhat, CPA, CIA, DBA, teaches accounting and exam prep at Farhat Lectures for CPA, CMA, and EA candidates and accounting students.