IFTA settles the gap between where you burned fuel and where you bought it. Your average fleet MPG turns each jurisdiction's miles into gallons burned there; subtract the gallons you bought there; multiply the difference by that jurisdiction's rate for the quarter. Owe on the shortfall, get credited on the surplus.

IFTA Fuel Tax Calculator

Work out what you owe (or get credited) in one jurisdiction for the quarter. Enter your fleet totals to get average MPG, then that jurisdiction's miles, the gallons you bought there, and its tax rate from the current IFTA rate matrix.

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What IFTA actually is, and why the math looks backwards

The International Fuel Tax Agreement exists to solve a problem created by the fact that you pay diesel tax at the pump but consume the fuel across state lines. Without it, a carrier would owe a separate return to every jurisdiction it touched. Instead you file one return with your base jurisdiction, which redistributes the money to the others on your behalf.

The part that surprises people is that IFTA does not care where you bought your fuel when it decides what you consumed. It works out consumption from your MILES. Run 3,200 miles through a state and, at a fleet average of 6.4 MPG, IFTA says you burned 500 gallons there — whether you fuelled up there or not. Your receipts only enter the calculation on the other side, as the gallons you already paid tax on in that jurisdiction. The return settles the difference.

This is why fuel-buying strategy has a tax consequence. Buy heavily in a low-tax state and run the miles in a high-tax one, and you will owe the high-tax state on the fuel you burned there. The tax follows the miles, not the pump.

The method, step by step

  1. Average fleet MPG = total miles in all jurisdictions ÷ total gallons purchased in all jurisdictions, for the quarter. One figure for the whole return.
  2. Taxable gallons for a jurisdiction = that jurisdiction's miles ÷ average fleet MPG. The fuel you consumed there.
  3. Tax-paid gallons = the gallons you actually bought in that jurisdiction, tax already collected at the pump.
  4. Net taxable gallons = taxable gallons − tax-paid gallons. Positive means you under-bought there.
  5. Net tax = net taxable gallons × that jurisdiction's rate for the quarter. Positive is owed; negative is a credit.

Repeat for every jurisdiction you ran in; the return is the sum of those lines. One implementation detail worth knowing: this calculator does not round your MPG before dividing by it. Rounding 7.142857 MPG to 7.14 first would shift the taxable gallons and therefore the tax, so the displayed MPG is rounded for reading while the full-precision value drives the arithmetic.

What it does not do: it does not add surcharge-state lines, it does not handle non-diesel fuel types with separate rate schedules, it does not compute penalties or interest on a late return, and it does not know your base jurisdiction's rounding conventions. Those are the reasons the number here is a check on your return rather than the return itself.

Worked examples

IFTA questions, answered

How is IFTA tax calculated?
In four steps, per jurisdiction. First, average fleet MPG: total miles in all jurisdictions divided by total gallons purchased in all jurisdictions for the quarter. Second, taxable gallons for a jurisdiction: the miles you ran there divided by that average MPG — the fuel you actually consumed there. Third, subtract the tax-paid gallons you bought there. Fourth, multiply the difference by that jurisdiction's tax rate for the quarter. A positive result is tax owed; a negative result is a credit because you already paid tax on fuel you carried out of the state.
Why does this calculator ask me for the tax rate?
Because IFTA tax rates are set by each jurisdiction and change every quarter, and a calculator with rates baked into it is wrong within a season. The only authoritative source is the International Fuel Tax Association (IFTA, Inc.) quarterly tax rate matrix. Look up your jurisdiction's rate for the quarter you are filing and enter it, and the arithmetic here — which does not go stale — does the rest. Any tool that shows you a rate without telling you which quarter it came from should be treated with suspicion.
What are taxable gallons versus tax-paid gallons?
Taxable gallons are the fuel you BURNED in a jurisdiction, which IFTA works out from your miles there and your fleet's average MPG — not from your receipts. Tax-paid gallons are the fuel you BOUGHT in that jurisdiction, where the tax was already collected at the pump. IFTA exists to settle the difference: if you drove 500 gallons' worth of miles through a state but only fuelled 420 gallons there, you owe that state the tax on the missing 80.
Do I use one MPG for every jurisdiction?
Yes. IFTA uses a single average fleet MPG for the whole quarter, derived from your total miles and total gallons across all jurisdictions. You do not calculate a separate MPG per state. That is why keeping accurate total mileage and every fuel receipt matters: an error in either total moves the taxable gallons for every jurisdiction on the return at once.
What about surcharge states?
A few jurisdictions — Indiana, Kentucky, and Virginia among them — charge an additional per-gallon surcharge that appears as its own line on the return, calculated on the gallons consumed in that state rather than netted against fuel purchases. This calculator handles the standard net-tax line and does not add a surcharge line for you. Check the rate matrix for whether your jurisdiction carries a surcharge and add it separately.
Is this a filed IFTA return?
No. This is a worksheet that reproduces the standard IFTA apportionment arithmetic so you can check a jurisdiction's number or sanity-check what your software produced. Your actual return has to be filed with your base jurisdiction on their form or portal, and your base jurisdiction's figures govern. Keep your mileage records and fuel receipts: those, not this page, are what an audit examines.

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