How to calculate IFTA by hand, step by step

Updated: November 14, 2026

IFTA math is simple once you see the steps. You need, for the quarter: miles driven in each state, and gallons of fuel you bought in each state (with tax paid at the pump).

Step 1: Your average MPG

MPG = total miles in all states / total gallons bought

Step 2: Taxable gallons per state

Taxable gallons = miles in that state / MPG. This is how much fuel you "used" in that state.

Step 3: Net taxable gallons

Net = taxable gallons - tax-paid gallons you bought in that state. A negative number means you bought more fuel there than you used, so you get a credit for that state.

Step 4: Tax per state

Tax = net gallons x that state's rate for the quarter. Rates change every quarter, so use the official IFTA tax rate matrix for the quarter you are filing.

Step 5: Add it up

The total of all states is what you owe (or get back) on your return to your base state.

Due dates

Returns are due the last day of the month after the quarter. If that day is a weekend or legal holiday, the next business day counts. A late or missing return can cost a penalty of $50 or 10% of the tax due, whichever is greater. Keep your mileage and fuel records for at least four years.

Our free calculator does steps 1 to 5 for you when you type in miles, gallons and rates per state.

Estimates for planning only, not tax or legal advice.