Introduction
Cost of Goods Sold sounds like a simple, factual number — what did the inventory actually cost — but for any business holding inventory across multiple purchase batches at different prices, "what did it cost" has three legitimate, genuinely different answers depending on which accounting method is chosen. This guide covers how FIFO, LIFO, and weighted average actually work, and why the choice between them is a real financial decision, not just a technicality.
Note: This is educational information, not accounting or tax advice. Inventory valuation method selection has real, ongoing tax and financial statement consequences and generally requires formal approval to change — confirm the right choice for your specific business with a qualified accountant.
Table of Contents
- Why the Same Inventory Produces Different Numbers
- FIFO: First In, First Out
- LIFO: Last In, First Out
- Weighted Average Cost
- A Worked Example Across All Three
- The LIFO/IFRS Restriction
- Why You Can't Switch Methods Casually
- Which Method Fits Your Business
- FAQ
- Conclusion
Why the Same Inventory Produces Different Numbers
A business that buys the same product at three different prices over time — because supplier costs rose — faces a genuine question when it sells a unit: which specific purchase cost gets assigned to that sale? The physical inventory doesn't answer this on its own (especially for interchangeable goods where individual units genuinely can't be tracked back to a specific purchase batch). That's exactly what FIFO, LIFO, and weighted average each answer differently.
FIFO: First In, First Out
FIFO assumes the oldest inventory purchased is the first sold. In a period of rising costs, this means COGS is calculated using the older, lower-cost inventory, leaving the newer, higher-cost inventory still on the books — which produces higher reported gross profit (and correspondingly higher tax) compared to the other methods during inflationary periods.
FIFO is also the method that most closely matches the actual physical flow of inventory for most businesses — perishable goods, fashion/trend-sensitive products, and most retail and e-commerce operations genuinely do sell older stock before newer stock arrives, making FIFO both the most common default and the most intuitively defensible choice for many small businesses.
LIFO: Last In, First Out
LIFO assumes the newest inventory purchased is the first sold — the reverse of FIFO. In a period of rising costs, this means COGS is calculated using the most recent, higher-cost inventory, which produces lower reported gross profit (and lower tax) compared to FIFO during the same inflationary period.
LIFO rarely reflects actual physical inventory flow — few businesses genuinely sell their newest stock first — but it exists specifically as a tax-advantaged accounting election available under US GAAP, used deliberately by businesses seeking to reduce taxable income during periods of rising costs, accepting the tradeoff of a less intuitive balance sheet inventory value in exchange.
Weighted Average Cost
Weighted average calculates a single blended cost per unit across all inventory on hand, recalculated as new purchases come in, and applies that average cost uniformly to every sale — regardless of which specific batch a given unit actually came from.
This method is particularly well-suited to businesses with large volumes of interchangeable, non-perishable inventory (raw materials, commodities, bulk goods) where tracking individual purchase batches through to specific sales isn't practical or meaningful. It also produces results that sit between FIFO and LIFO during periods of changing costs, since it blends old and new costs together rather than assigning one extreme or the other.
A Worked Example Across All Three
Scenario: a business purchases 100 units at $10 each, then later purchases 100 more units at $14 each (costs rose). It then sells 100 units.
| Method | COGS for the 100 Units Sold | Ending Inventory Value |
|---|---|---|
| FIFO | $1,000 (uses the $10 batch first) | $1,400 (100 units at $14 remain) |
| LIFO | $1,400 (uses the $14 batch first) | $1,000 (100 units at $10 remain) |
| Weighted Average | $1,200 (blended $12/unit average) | $1,200 |
Same inventory, same sale — three different COGS figures, directly producing three different reported gross profit numbers for the identical underlying transactions. This is exactly why the method chosen isn't a minor technical detail.
The LIFO/IFRS Restriction
This is a genuinely important constraint worth knowing: LIFO is permitted under US GAAP but is not permitted under IFRS (International Financial Reporting Standards), the accounting framework followed by most countries outside the US. A US business using LIFO that later needs IFRS-compliant statements — through foreign investment, an international listing, or a foreign parent/subsidiary relationship — would need to address this restriction directly, since LIFO simply isn't an available option under IFRS.
Why You Can't Switch Methods Casually
Changing inventory valuation methods is a formal accounting change, not a yearly optimization decision. In the US, this generally requires filing Form 3115 with the IRS, and once adopted, the method must be applied consistently going forward. This isn't designed to be revisited casually based on which method happens to look more favorable in a given year — it's meant to be chosen deliberately, with genuine reasoning, and maintained.
Which Method Fits Your Business
- FIFO — the most common default for small businesses, especially those selling physical, perishable, or trend-sensitive goods, since it typically matches actual inventory flow reasonably well
- Weighted average — often preferred for businesses with large volumes of interchangeable, non-perishable inventory where individual batch tracking isn't practical
- LIFO — less common for small businesses, given its complexity, its unavailability under IFRS, and the fact that it rarely reflects actual physical inventory movement — but a legitimate consideration specifically for tax planning purposes in the right circumstances, worth discussing with an accountant rather than dismissing outright
FAQ
FIFO (First In, First Out) assumes the oldest inventory purchased is the first sold. LIFO (Last In, First Out) assumes the newest inventory purchased is the first sold. Weighted average calculates a single blended cost per unit across all inventory on hand, regardless of purchase order, and applies that average cost to every sale. All three are legitimate methods for the same underlying transactions — they simply assign cost differently, which changes reported COGS and profit.What's the difference between FIFO, LIFO, and weighted average?
Because during periods of changing costs — especially rising costs, which is the common scenario — each method assigns a different cost figure to the units sold, which directly changes reported cost of goods sold and therefore reported gross profit, even though the physical inventory and sales transactions are exactly the same. This is a genuine accounting choice with real financial statement and tax consequences, not just a technical formality.Why does the choice of COGS method actually matter if the inventory and sales are identical?
No — LIFO is permitted under US GAAP but is not permitted under IFRS (International Financial Reporting Standards), which most countries outside the US follow. This makes LIFO unavailable to businesses reporting under IFRS, and it's a genuine consideration for any US business that might eventually need IFRS-compliant statements, such as through foreign investment or an international listing.Is LIFO allowed everywhere?
LIFO generally produces the lowest reported profit — and therefore the lowest tax bill — during periods of rising costs, because it assigns the most recent, highest-cost inventory to COGS first, leaving older, lower-cost inventory on the balance sheet. This tax advantage is precisely why LIFO exists as an accounting option in the US despite not reflecting the actual physical flow of goods for most businesses.Which COGS method results in the lowest tax bill during inflation?
No — changing inventory valuation methods is a formal accounting change that generally requires IRS approval (via Form 3115 in the US) and must be applied consistently going forward once adopted. This isn't a decision to revisit casually based on which method looks more favorable in a given year; it needs to be chosen deliberately and applied consistently.Can a business switch between COGS methods whenever it wants?
FIFO is the most common default for small businesses, particularly those selling physical, perishable, or trend-sensitive goods, since it usually reflects the actual physical flow of inventory reasonably well (older stock typically does sell before newer stock in most retail and e-commerce operations). Weighted average is often preferred for businesses with large volumes of interchangeable, non-perishable inventory where tracking individual purchase batches isn't practical. LIFO is less common for small businesses given its complexity and its unavailability under IFRS.Which method should a small business actually use?
Selling on Amazon or Shopify specifically? See our guide on e-commerce accounting, which covers COGS in that specific context.
Conclusion
The businesses that get burned by COGS methodology aren't usually the ones that chose wrong — they're the ones that never chose deliberately at all, defaulting to whatever their accounting software set up automatically without understanding what that choice actually means for reported profit and tax. Given how binding the decision becomes once made, it's worth a genuine conversation with an accountant before the first inventory purchase, not a retroactive fix once the wrong method has already shaped a year of financial statements.
If you'd like help choosing and implementing the right COGS method for your inventory, get in touch for a free consultation.