How IFTA actually works
The International Fuel Tax Agreement exists so you file one quarterly return instead of forty-eight. The idea is simple even if the paperwork isn't: every state gets fuel tax on the miles you drove inside it, regardless of where you actually bought the diesel.
That's the part that trips people up. You pay fuel tax at the pump in the state where you fuel. But IFTA taxes where you burn the fuel. So if you buy cheap fuel in one state and drive through three others, you owe those three and get credit for what you overpaid in the first. It all nets out on one form.
The four-step math
- Fleet MPG = total miles ÷ total gallons purchased, across every state.
- Taxable gallons per state = miles driven there ÷ fleet MPG. This is the fuel you burned in that state.
- Tax due per state = taxable gallons × that state's tax rate.
- Net per state = tax due − tax already paid at the pump (gallons bought there × rate). Add up every state for your net.
The calculator runs all four steps as you type. A positive net means you owe; a negative net means you overpaid and are due a credit.
Records you need to keep
- Miles by jurisdiction — from your ELD, trip sheets, or GPS. Every mile counts, loaded or empty.
- Fuel receipts — date, location, gallons, and price, for every purchase.
- Your quarterly totals — keep them at least four years in case of audit.
Your cost per mile already carries fuel as your biggest line, and IFTA is part of what makes fuel expensive. Fueling smart helps both numbers at once.