IFTA Calculator

The formula, written out

IFTA is not complicated. What trips people up is the order: fleet MPG comes from the entire quarter across every jurisdiction, and only then is each jurisdiction settled against that single figure. Working it out state by state in isolation gives the wrong answer every time.

1. Fleet MPG = total miles in all jurisdictions ÷ total gallons purchased in all jurisdictions.
2. Taxable gallons in a jurisdiction = taxable miles there ÷ fleet MPG. This is the fuel you are treated as having burned there.
3. Net taxable gallons = taxable gallons − tax-paid gallons you actually bought there.
4. Tax due = net taxable gallons × that jurisdiction's rate. A negative figure is a credit.
5. Surcharge, where it applies = taxable gallons × the surcharge rate, with no credit for purchases.
6. Net position = everything added together. Credits in one jurisdiction offset tax owed in another.

The idea underneath is simple. You pay fuel tax where you burn the fuel, not where you buy it. Buying cheap in a low-tax state and running the miles in a high-tax state does not avoid anything — it just moves the payment from the pump to the quarterly return.

A worked example

A truck runs 9,000 miles in a quarter and buys 1,200 gallons. Fleet MPG is 9,000 ÷ 1,200 = 7.50.

In one jurisdiction it ran 1,800 miles and bought 100 gallons. Taxable gallons there are 1,800 ÷ 7.50 = 240. Net taxable gallons are 240 − 100 = 140. At a rate of $0.29 that is $40.60 owed.

In another it ran 900 miles and bought 300 gallons — a cheap fuel stop on a short leg. Taxable gallons are 900 ÷ 7.50 = 120. Net is 120 − 300 = −180, so at $0.24 that is a credit of $43.20.

Those two net out to a small credit before the rest of the quarter is added. That is the entire mechanism, repeated across every jurisdiction you ran in.

The records the arithmetic depends on

A calculator is only as good as what goes into it, and IFTA record-keeping is where audits are won or lost. Distance records need to show, per trip: the date, where it started and ended, the route, beginning and ending odometer readings, total trip miles, the miles broken down by jurisdiction, and which vehicle and licensee it belonged to.

Fuel records need the date, the seller's name and location, the number of gallons, the fuel type, the price or total, and the vehicle it went into. A receipt missing the vehicle number is a receipt an auditor can disallow, which turns tax-paid gallons into gallons you cannot claim credit for.

What to keep, for how long, and what auditors actually look at is covered in the IFTA filing guide, along with the four quarterly deadlines and what happens when one is missed.

Where the numbers usually go wrong

• Mixing up miles and gallons columns. It sounds implausible until you have typed fifty rows. An implausible fleet MPG is usually this, and the calculator flags it.
• Counting fuel bought in a jurisdiction you did not run in. Gallons are claimed where they were purchased, but the miles have to exist. A fuel stop just over a state line with no miles recorded there stands out.
• Missing receipts. Every disallowed receipt reduces your tax-paid gallons and increases what you owe. Chase them before you file, not after.
• Forgetting a jurisdiction entirely. Short runs through a corner of a state still count. Total miles on the return should reconcile to total miles on your logs.
• Rounding early. Round at the end, not at each step. This calculator carries gallons to three decimals for that reason.
• Assuming Oregon works like everywhere else. Oregon runs a weight-mile tax rather than a diesel fuel tax, so its treatment on an IFTA return is not the same as its neighbours. Check the current instructions rather than pattern-matching.

Using the calculator

Add a row per jurisdiction, enter the miles you ran and the gallons you bought there, then the tax rate from the quarterly matrix. Each row shows its own taxable gallons, net gallons and amount as you type, and the summary at the bottom gives the fleet MPG and the net position. If a jurisdiction levies a surcharge, put that rate in the surcharge column and it is calculated on taxable gallons without credit, which is how surcharges work.

Export the CSV and open it in a spreadsheet to check line by line against what you are about to file. The file includes the fleet MPG and every intermediate figure, so a disagreement with your return is quick to trace.

Nothing is uploaded — the calculation and the CSV are both built in your browser, and unlike the document generators this one is not saved to an account on any plan. Nothing is kept between visits, so export before you close the tab.

Limitations

This is a working calculation, not a filed return and not tax advice. It does not know your base jurisdiction's rules, does not handle jurisdiction-specific exemptions, does not apply split-rate periods where a rate changed mid-quarter, and does not calculate interest or penalties on a late filing. It also cannot tell whether the miles and gallons you typed match your records.

File through your base jurisdiction, and check the arithmetic against their form before you submit.

Common questions