The fuel business has a quirk: the gallons in your tank are all mixed together, but how you value them on paper can change your margin and your tax bill. When fuel cost moves every day, the inventory method you choose moves the numbers. This explains what FIFO, LIFO, and average cost mean, in plain words, with the math worked out on one tank.
Why valuation matters
Because fuel cost changes constantly, the value you assign to the fuel in your tank flows into your cost of goods sold, your margin, and your taxable income. With prices moving daily, the method can meaningfully shift the numbers, so pick one on purpose rather than by default. It rides on accurate per-load cost, the same data behind your month-end close.
FIFO: first in, first out
FIFO assumes the oldest fuel is sold first, so the earliest purchase costs flow to cost of goods sold and the newest stay in inventory. In a rising market, FIFO tends to show lower cost of goods and higher reported profit, with inventory valued close to current cost.
LIFO: last in, first out
LIFO assumes the newest fuel is sold first, so recent costs flow to cost of goods sold and older costs stay in inventory. In a rising market, LIFO tends to show higher cost of goods and lower reported profit, which can reduce taxable income, while leaving inventory carried at older costs.
Average cost: the middle ground
Average cost blends all purchases into one average per gallon, used for both cost of goods and inventory. It smooths the swings between FIFO and LIFO and is simpler to follow when you are constantly buying at different prices, which is exactly the fuel situation. Many operators find it a practical middle path.
One tank, three loads: the math
Say you take three 8,000-gallon loads in a month at $2.40, $2.60, and $2.80 per gallon, then sell 16,000 gallons. Each method assigns a different cost to the gallons you sold:
- FIFO: the $2.40 and $2.60 loads go to cost of goods sold. COGS is $40,000, and the tank carries the $2.80 load at $22,400.
- LIFO: the $2.80 and $2.60 loads go to cost of goods sold. COGS is $43,200, and the tank carries the $2.40 load at $19,200.
- Average cost: all 24,000 gallons average to $2.60. COGS is $41,600, and the tank carries $20,800.
Same tank, same gallons, and a $3,200 swing in reported profit between FIFO and LIFO. At an illustrative 25 percent combined tax rate, that is $800 of tax timing on one month of buying. Scale it across a year of price moves and the method stops being a footnote.
LIFO is an election, with strings
LIFO requires an IRS election: you file Form 970 with the tax return for the first year you use it. The conformity rule in Section 472(c) then binds your books. If you report on LIFO for tax, the financial statements you give owners and lenders must use LIFO too. The election is also hard to unwind. Dropping LIFO is an accounting method change that needs IRS consent on Form 3115, and under Rev. Proc. 2015-13 you generally cannot re-elect LIFO for five years after leaving it.
Which to use
FIFO and average cost need no special permission and fit most small operators. LIFO can defer tax through a long rising market, and the election paperwork above is the price of entry. Weigh that trade with your accountant before the first return of the year you want it to apply.
Whatever you pick, the method is only as good as the cost data under it. Your system has to capture cost per load, apply the method the same way every month, and tie to your physical wet stock. Guesswork here distorts both margin and tax. FastDragon Books tracks fuel cost by purchase on double-entry accounting, so the $3,200 question above gets answered from load tickets instead of estimates.
Questions we hear a lot
What happens to LIFO when fuel prices fall?
The effect reverses. In a falling market the newest, cheapest loads are charged to cost of goods sold first, so a LIFO business reports more profit and more taxable income than a FIFO business would. LIFO defers tax while prices climb, and a long price slide gives some of that deferral back.
What is a LIFO layer liquidation?
It is what happens when year-end gallons drop below the prior year and old, low-cost LIFO layers get released into cost of goods sold. Those old costs can be decades cheap, so the release inflates taxable income in one shot. Fuel operators trigger this by letting tanks run lean across a fiscal year-end, which is one reason LIFO users watch December inventory levels closely.
Can I use one method for fuel and a different one for store merchandise?
Yes. Tax rules let you apply different inventory methods to different classes of goods as long as each class is handled consistently year over year. Convenience store operators commonly pair one method for fuel with the retail method for in-store merchandise.
Is LIFO allowed outside the United States?
No. IFRS, the accounting standard used in most countries, prohibits LIFO under IAS 2, while US GAAP and the IRS both permit it. This matters if a parent company, foreign lender, or potential buyer requires IFRS-basis financial statements, because a US LIFO business would have to restate inventory for them.