Guide
How to calculate IFTA fuel tax
IFTA looks complicated because it's five small steps repeated for every state you touched. Here they are in order, with a worked example.
Add up total miles for the quarter
Every mile the qualified vehicle ran, in every jurisdiction, including deadhead and personal miles. Keep them separated by state — that split is the whole return.
Add up total gallons purchased
Total taxed fuel bought during the quarter, again split by the state where you pumped it. Bulk fuel is tracked separately from over-the-road purchases.
Divide to get fleet MPG
Total miles ÷ total gallons. One average for the whole fleet and the whole quarter — not per truck, not per state. Most audits start here, so round the way your base jurisdiction tells you to and keep the raw numbers.
Work out taxable gallons per state
Miles in that state ÷ fleet MPG. That is the fuel the state says you burned inside its borders, regardless of where you bought it.
Net it against what you bought, then apply the rate
Taxable gallons − gallons purchased in that state = net gallons. Multiply by that state's current rate for your fuel type. Positive means you owe it; negative is a credit that offsets what you owe elsewhere.
Worked example
One truck, 12,000 total miles, 2,000 total gallons — so fleet MPG is 6.0.
| State | Miles | Taxable gal | Bought gal | Net gal |
|---|---|---|---|---|
| TX | 6,000 | 1,000 | 1,400 | -400 |
| OK | 3,000 | 500 | 300 | +200 |
| KS | 3,000 | 500 | 300 | +200 |
Multiply each net-gallon figure by that state's current rate and add the results. You buy most of your fuel in Texas, so Texas owes you a credit while Oklahoma and Kansas want their cut.
Build it in Excel or Google Sheets
Prefer a spreadsheet? One row per state. Put total miles in cell B10 and total gallons in C10 (use =SUM over the rows above), and the state's rate per gallon in column F.
| Column | Meaning | Formula (row 2) |
|---|---|---|
| A | State | TX |
| B | Miles | 6000 |
| C | Gallons bought | 1400 |
| D | Taxable gallons | =B2/($B$10/$C$10) |
| E | Net gallons | =D2-C2 |
| F | Rate per gallon | current quarter rate |
| G | Tax due / (credit) | =E2*F2 |
Sum column G for the net amount due. Remember Kentucky and Virginia add a surcharge on taxable gallons, and rates change every quarter — see current rates by state. Not sure which miles belong in column B? Read taxable miles for IFTA.
Deadlines that bite
Returns are due the last day of the month after each quarter ends: April 30, July 31, October 31 and January 31. A return is required even for a quarter where the truck never moved. Miss it and you face penalty plus interest per jurisdiction and, eventually, a suspended IFTA license.
Skip the arithmetic
Type your miles and gallons; the calculator does all five steps live.
Open the free calculatorGeneral information, not tax advice. Rules and rounding vary by base jurisdiction; confirm requirements with yours before filing.