IFTA, the International Fuel Tax Agreement, exists so you do not file fuel tax separately in every state you drive through. You file one quarterly return with your base state, and the states settle up between themselves based on where you actually drove.
How the math works
First comes your fleet MPG for the quarter: total miles divided by total gallons purchased, everywhere. Not per state, and not per truck. The whole quarter, one number.
Then for each state: the miles you ran there, divided by that fleet MPG, gives your taxable gallons for that state. Compare that against the gallons you actually bought there. Burn more than you bought and you owe that state the difference in tax. Buy more than you burned and you get a credit.
That is the entire mechanism. The reason it feels complicated is the record-keeping, not the arithmetic.
Why rates are not built into any calculator worth trusting
IFTA rates change every quarter and several jurisdictions carry surcharges on top. Any tool that hard-codes them is wrong within three months and will not tell you so. Pull the current rates from the official IFTA rate sheet each quarter.
The habit that saves your weekend
Record your odometer at every state line crossing and keep every fuel receipt. That is it. Do it and filing takes twenty minutes. Skip it and you spend two days reconstructing a quarter from memory and credit card statements, and you will still get it wrong.
Our IFTA calculator does the gallons side for you: enter miles and gallons by state and it works out your fleet MPG, the taxable gallons, and what each state owes or credits once you add the current rate.
