How to calculate IFTA tax
The calculation is five steps and one division. What goes wrong is almost never the arithmetic — it is which numbers get used, and in what order. Here is the whole thing with a worked example using real published rates.
Rates in the example are from the 4Q2026 IFTA matrix. Your quarter's rates will differ.
What you need before you start
Four sets of numbers, and nothing else:
- Total miles travelled in the quarter by qualifying vehicles.
- Total gallons of fuel purchased in the quarter.
- Miles per jurisdiction — how far you drove in each state or province.
- Gallons purchased per jurisdiction — where you actually bought fuel.
Note what is not on that list: the rate you paid at the pump, and the tax already included in that price. IFTA does not recalculate what you paid. It reallocates tax between jurisdictions based on where the fuel was burned versus where it was bought.
The five steps
1. Fleet average mpg
mpg = total miles ÷ total gallons purchased
This is a fleet-wide figure, not a per-truck one. It is the single most consequential number in the return, because every jurisdiction's gallons are derived from it.
2. Gallons taxed in each jurisdiction
gallons taxed = miles in jurisdiction ÷ fleet mpg
This is an estimate of how much fuel you burned there. It is not measured, it is inferred from distance and average economy.
3. Net gallons in each jurisdiction
net gallons = gallons taxed − gallons purchased there
Positive means you burned more than you bought there — you owe that jurisdiction. Negative means you bought more than you burned — that jurisdiction owes you.
4. Tax per jurisdiction
tax = net gallons × that jurisdiction's rate for the quarter
5. Net position
net due = sum of all jurisdictions
Positive total: you pay. Negative total: you are refunded. The jurisdictions are not settled separately — they net against each other into one figure.
A worked example
One truck, one quarter, three states. 2,000 total miles and 310 total gallons purchased.
| Jurisdiction | Miles | Gallons bought | Gallons taxed | Net gallons | Rate | Tax |
|---|---|---|---|---|---|---|
| Texas | 1,400 | 100.00 | 217.00 | 117.00 | $0.2000 | +$23.40 |
| New Mexico | 400 | 100.00 | 62.00 | −38.00 | $0.2100 | −$7.98 |
| California | 200 | 110.00 | 31.00 | −79.00 | $0.9790 | −$77.34 |
| Net position | −$61.92 | |||||
Fleet mpg is 2,000 ÷ 310 = 6.45 mpg. Texas gets 1,400 ÷ 6.45 = 217.00 gallons taxed against 100 gallons bought, so 117 gallons of tax is owed there. California is the mirror image: only 200 miles driven but 110 gallons bought, so 79 gallons of California tax was paid and not burned there.
The net is a refund of $61.92 — not because the quarter was profitable, but because the fuel was bought in the most expensive jurisdiction and burned elsewhere. Run the same quarter buying in Texas and driving in California and the number flips hard in the other direction.
The mistakes that actually break returns
Using the wrong mpg
Fleet average is the default, but some jurisdictions permit or require per-vehicle mpg. If your trucks differ sharply in economy, the two methods give materially different answers. Confirm which applies to your licence before you file, not after.
Leaving out empty miles
Deadhead burns fuel in a jurisdiction exactly as loaded miles do. Miles are miles. A return built only on loaded miles understates gallons burned everywhere and will not reconcile against your fuel records.
Forgetting that rates are quarterly
Rates are reset each quarter, and jurisdictions can issue mid-quarter notices — holidays, split-quarter rates that apply to one fuel but not another, temporary relief. A rate table without a quarter on it is not usable. Check the quarter on your return against the quarter on the table.
Inventing a rate where none exists
Some jurisdictions publish no rate for a given fuel type. Oregon levies no IFTA fuel tax at all and uses a weight-mile tax instead. If you fill in a number for a jurisdiction that reports none, your return will not match the official matrix.
Mixing fuel types
Diesel and gasoline have separate rates and are reported separately. A mixed fleet needs two calculations, not one blended average.
Do the arithmetic once, check it twice
The IFTA calculator runs exactly these five steps against the current quarter's rates and shows the per-jurisdiction breakdown, so you can compare it line by line against your own records. The full rate table is on the IFTA tax rates page, and the official matrix is at iftach.org.
Use the calculator as a cross-check, not as the return. Your records are the source of truth, and the official matrix is the authority on rates.
Questions
Do I calculate IFTA per truck or for the whole fleet?
The standard method uses a fleet average mpg across all qualifying vehicles under the licence. Some jurisdictions allow or require per-vehicle mpg instead. If you run vehicles with very different fuel economy, ask your base jurisdiction which method applies to you before you file.
Are deadhead miles included in IFTA miles?
Yes. IFTA miles are all miles travelled by a qualifying vehicle, loaded or empty. Empty miles burn fuel in a jurisdiction just as loaded miles do, so they belong in the calculation.
Why do I get a refund instead of owing tax?
Because you bought more fuel in a jurisdiction than you burned there. Fuel tax is paid at the pump, so buying fuel in a high-rate jurisdiction and burning it in a low-rate one puts you in a refund position. The reverse puts you in a payment position.
What if a jurisdiction publishes no rate for my fuel type?
Then there is no IFTA fuel tax to settle for that fuel in that jurisdiction. Oregon is the common example: it does not levy an IFTA fuel tax and applies a weight-mile tax instead. Do not substitute another jurisdiction's rate.
When is the return due?
Generally by the last day of the month following the end of the quarter, though your base jurisdiction sets the exact date and some allow electronic filing later. Check your base jurisdiction's schedule rather than relying on a general rule.