Worked example
A quarter across three states: 3,000 total miles on 400 gallons bought, which makes fleet economy 7.5 miles per gallon.
- Illinois: 1,200 miles ÷ 7.5 = 160 taxable gallons, minus 100 bought there = 60 net. At $0.626 that is +$37.56 owed.
- Indiana: 900 ÷ 7.5 = 120 taxable, minus 150 bought = −30 net. At $0.570 that is −$17.10, a credit.
- Ohio: 900 ÷ 7.5 = 120 taxable, minus 150 bought = −30 net. At $0.470 that is −$14.10, a credit.
- Net for the quarter: 37.56 − 17.10 − 14.10 = $6.36 owed.
Notice how small the final number is against the gross figures. That is normal. IFTA is a reallocation, not a new tax, so a driver who buys fuel roughly where they burn it ends up near zero either way.
The formula
Fleet MPG = total miles ÷ total gallons bought Taxable gallons = miles in that jurisdiction ÷ fleet MPG Net gallons = taxable gallons − gallons bought there Tax for that row = net gallons × that jurisdiction's rate Quarter total = sum of every row, credits included
Negative rows are kept as negatives. A credit in one state genuinely offsets tax owed in another, and clamping it to zero would overstate what you owe.
Why the rates are not built in
IFTA rates are republished every quarter, and several jurisdictions adjust theirs on an index. Any rate hard-coded into a calculator is wrong within months, and a wrong rate is worse than no rate because it looks authoritative.
So the rate column is yours to fill from the current rate sheet. It also means this tool works for surcharge jurisdictions and for provinces, where a single built-in table would not.
Records that survive an audit
IFTA jurisdictions audit, and the return itself is not evidence. What you need is the underlying record: distance by jurisdiction for every trip, and a receipt for every gallon you are claiming.
Distance records normally come from an ELD or GPS system and must show date, route, and miles in each jurisdiction. Fuel receipts must show the date, the seller, the gallons, and the vehicle — a credit card statement alone is not enough, because it does not prove which truck was fuelled.
The usual finding in an audit is not fraud, it is missing paperwork. Where records are inadequate, an auditor can assess using a standard fuel economy, often around 4 miles per gallon, which is far worse than any real truck achieves and produces a much larger bill. Keeping the receipts is cheaper than arguing.
Common mistakes
- Using actual MPG per state. IFTA uses one fleet average across every jurisdiction, not what the truck did in each one.
- Leaving out states where you bought no fuel. Miles driven there are taxable whether or not you fueled there.
- Dropping states where you drove no miles. Fuel bought there still earns a credit.
- Mixing quarters. Miles, gallons and rates all have to come from the same filing period.
- Forgetting surcharge jurisdictions. A few states levy a separate surcharge line that is calculated on taxable gallons rather than net.
FAQ
How is IFTA tax calculated?
Divide total miles by total gallons bought everywhere to get your fleet MPG. For each jurisdiction, divide its miles by that MPG to get taxable gallons, subtract the gallons you bought there, and multiply the difference by that jurisdiction rate.
Why do I enter the tax rates myself?
IFTA rates change every quarter. Any rate built into a calculator is stale within months, so this tool takes the numbers straight off the rate sheet for the quarter you are filing.
What does a negative number mean?
A credit. You bought more fuel in that jurisdiction than you burned there, so you already paid tax on gallons you used elsewhere. Credits offset what you owe in other jurisdictions.
What is fleet MPG and why does it matter?
It is your total miles divided by your total gallons for the quarter, across every jurisdiction. IFTA uses one average for the whole fleet rather than actual consumption per state, which is why buying fuel in a low-tax state still helps.
Can I file with this calculator?
No. This is an estimate for your own records and for checking a return before you submit it. Your official filing goes to your base jurisdiction on their forms.
Rules last checked: Sep 24, 2026
Estimate only. Confirm with your carrier before booking.