How to calculate your IFTA quarterly fuel tax, step by step
IFTA works out fuel tax by where you drove, not where you bought fuel. Each quarter you divide total miles by total gallons to get fleet MPG, turn each jurisdiction's miles into taxable gallons, subtract the gallons you already paid tax on at the pump there, and multiply what is left by that jurisdiction's rate. Positive means tax due; negative means a credit. Below is the method with numbers from the Fuelfern example file (made-up trips and made-up round rates, so you can follow the maths).
Example file, made-up quarter: fleet MPG 7.14 (total miles / total gallons). Rates are made-up round numbers.
| Jurisdiction | Taxable miles | Taxable gallons | Tax-paid gallons | Net taxable gallons | Example rate | Tax due / (credit) |
|---|---|---|---|---|---|---|
| Illinois | 4,465 | 625.35 | 0.00 | 625.35 | $0.70 | $437.75 |
| Indiana | 5,381 | 753.64 | 1,431.60 | -677.96 | $0.60 | ($406.78) |
| Kentucky | 1,363 | 190.90 | 1,097.40 | -906.50 | $0.30 | ($271.95) |
Source: Return tab of the Fuelfern example file (products/fuelfern/src/workbook.py, made-up rates EXRATE; published screenshot https://cybermaxtools.com/store/img/fuelfern-1.webp). Surcharges omitted here.
Why buying fuel in a state is not the same as paying its tax
In the example, the trucks drove 4,465 miles in Illinois but bought no fuel there. At 7.14 MPG that is 625.35 gallons burned in Illinois with no Illinois tax paid, so 625.35 x the example rate of 0.70 gives 437.75 due.
In Indiana they drove 5,381 miles (753.64 gallons burned) but bought 1,431.60 gallons there, paying Indiana tax at the pump. Net taxable gallons are -677.96, so Indiana owes the carrier a credit: 677.96 x 0.60 = 406.78.
Where the numbers come from
Miles by jurisdiction come from trip sheets, ELD or GPS reports; tax-paid gallons from fuel receipts (bulk fuel needs its own records). Rates and surcharges come from the IFTA tax rate matrix, published free each quarter at iftach.org.
Keep the records: IFTA audits ask for them, and jurisdictions expect them to be kept for four years.
When to file
Returns are due the last day of the month after each quarter: 30 April, 31 July, 31 October and 31 January. You file one return with your base jurisdiction, which shares the money with the others. If you had no IFTA miles in a quarter you still file.
Fuelfern vs the alternatives
Published prices, each checked on the date shown; prices change, so confirm on each site.
| Product | Price | Free | Checked |
|---|---|---|---|
| Fuelfern IFTA Quarterly Calculator | $19 once, every quarter | free preview PDF | live |
| IFTAplus | Standard $4.99/month (US); quarterly filing preparation $39 for 1-2 trucks | 2026-10-10 | |
| TenTrucks (TMS with IFTA) | Basic $145/month for up to 5 trucks | 2026-10-10 |
Why teams pick Fuelfern
- The same five steps apply in every IFTA jurisdiction; only the rates change each quarter.
- Doing it by hand for 3-4 states is fine; Fuelfern repeats it for all 58 with checks for missing rates.
- Rates come from the free IFTA tax rate matrix at iftach.org; never use the example's made-up rates.
How it works
- Total your fleet's miles in all jurisdictions (IFTA and non-IFTA) and all gallons bought for the quarter.
- Fleet MPG = total miles / total gallons (7.14 in the example).
- For each jurisdiction: taxable gallons = taxable miles there / fleet MPG.
- Net taxable gallons = taxable gallons - tax-paid gallons you bought there.
- Tax due (or credit) = net taxable gallons x that jurisdiction's rate for the quarter, plus any surcharge; add up all lines for the total.
FAQ
What MPG do I use?
Your fleet's actual MPG for the quarter: total miles in all jurisdictions divided by total gallons bought. Do not use a guessed or rounded figure.
Why not just use IFTA software?
If you want GPS mileage and filing help, software is worth paying for ($4.99/month at IFTAplus, $145/month in TenTrucks). If you keep your own trip sheets and receipts, the maths above is all there is, and Fuelfern does it for all 58 jurisdictions for $19 once.
Can a jurisdiction show a credit?
Yes: when you bought more tax-paid fuel there than you burned there, the line is negative. Credits offset tax due elsewhere on the same return.
Is this tax advice?
No. It explains the standard IFTA calculation; check your base jurisdiction's instructions.