How IFTA fuel tax is calculated, step by step
Short answer
Divide total miles by total gallons for the quarter to get your fleet MPG. For each jurisdiction, taxable gallons = miles driven there ÷ fleet MPG. Tax due there = taxable gallons × that jurisdiction's rate for the quarter. Subtract the tax you already paid on gallons bought there (gallons purchased × rate). Positive totals are owed, negative are credits; the net is your IFTA payment or refund.
Copy-paste code
Spreadsheet (one row per jurisdiction: B miles, D gallons bought, E rate)
Fleet MPG: =SUM(B:B) / SUM(D:D)
Taxable gallons: =ROUND(B2 / FleetMPG, 0)
Net tax: =(C2 - D2) * E2
Total due: =SUM(F:F)| Jurisdiction | Miles | Taxable gal. | Gal. bought | Rate | Net tax |
|---|---|---|---|---|---|
| A | 12,000 | 2,000 | 2,600 | $0.30 | -$180.00 (credit) |
| B | 10,000 | 1,667 | 1,400 | $0.40 | +$106.80 |
| C | 8,000 | 1,333 | 1,000 | $0.35 | +$116.55 |
| Total | 30,000 | 5,000 | 5,000 | +$43.35 owed |
Source: example numbers; net = (taxable - bought) × rate
Good to know
Use the rates from the IFTA tax rate matrix for that quarter; some jurisdictions add surcharges.
General information, not tax advice; your base jurisdiction's instructions decide rounding and forms.
Fuelfern IFTA Quarterly Calculator: Your IFTA quarter, worked out by jurisdiction
from $19Fuelfern works out your IFTA quarter by jurisdiction in a ready spreadsheet, with the trip and fuel logs it needs.
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When are IFTA returns due?
The last day of the month after each quarter: April 30, July 31, October 31 and January 31.
Do I need receipts?
Yes, fuel receipts and trip records are the evidence in an audit.
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