How to Calculate IFTA Fuel Tax: A Worked Example
How the numbers are worked out
- Average MPG = all miles in all jurisdictions (IFTA and non-IFTA) ÷ all fuel put in the trucks, rounded to 2 decimals.
- Taxable gallons per jurisdiction = taxable miles there ÷ average MPG.
- Net taxable gallons = taxable gallons − tax-paid gallons bought there.
- Tax = net gallons × that quarter's rate. A negative result is a credit.
- Surcharge (Kentucky, Virginia) = taxable gallons × the surcharge rate, with no tax-paid credit.
That's the method in the IFTA-13 return instructions: miles and gallons as whole numbers and MPG to 2 decimals.
Worked example (Q3 2026 rates)
One truck drove 30,000 miles: 12,000 in Texas, 9,000 in Oklahoma and 9,000 in Kentucky. It bought 5,000 gallons of tax-paid diesel: 2,500 in Texas, 1,000 in Oklahoma and 1,500 in Kentucky. Average MPG = 30,000 ÷ 5,000 = 6.00.
| Jurisdiction | Miles | Taxable gal | Tax-paid gal | Net gal | Rate | Tax / (credit) |
|---|---|---|---|---|---|---|
| KY | 9,000 | 1,500 | 1,500 | 0 | 0.220 | 0.00 |
| KY surcharge | 1,500 | 0 | 1,500 | 0.105 | 157.50 | |
| OK | 9,000 | 1,500 | 1,000 | 500 | 0.190 | 95.00 |
| TX | 12,000 | 2,000 | 2,500 | -500 | 0.200 | −100.00 |
| Total due | 152.50 | |||||
Texas is a credit because more fuel was bought there than burned there. Kentucky's tax nets to zero, but the surcharge still applies to all 1,500 taxable gallons.
Last reviewed 2026-10-27. General information, not tax advice. The IFTA Articles of Agreement, the Procedures Manual and your base jurisdiction decide what applies.