IFTA settles the difference between where you bought fuel and where you burned it. The arithmetic is simple once the three inputs are clear.
Step 1 — fleet MPG for the quarter
fleet MPG = total miles ÷ total gallons purchased
This is a whole-quarter figure across every jurisdiction, not a single trip. Getting it wrong shifts every jurisdiction’s number.
Step 2 — taxable gallons per jurisdiction
taxable gallons = miles in jurisdiction ÷ fleet MPG
This is the fuel you are treated as having burned there, regardless of where you actually bought it.
Step 3 — net tax per jurisdiction
net tax = (taxable gallons − gallons bought there) × rate
A positive number is tax owed. A negative number is a credit, because you bought more fuel in that jurisdiction than you used there.
Where the rates come from
Rates change every quarter and are published by IFTA, Inc. Take them from the official matrix. Third-party tax sites go stale, and a stale rate produces a wrong filing.
What this does not cover
Surcharges, interest, penalties and jurisdiction-specific adjustments are outside the basic calculation. So is filing itself, which happens through your base jurisdiction.