Reference

How an IFTA return is actually calculated

The arithmetic is not complicated. What catches people is the rounding, the one column that behaves differently from the others, and a surcharge rule that is easy to get backwards in a way that understates what you owe.

This is the same sequence CabPurser's calculator runs. It is written out here so you can check it against your own return rather than take a number on trust.

Step one

Your fleet's average miles per gallon

Everything downstream hangs off this number, and it is fleet-wide — not per jurisdiction.

Total miles
Ending odometer minus starting odometer, for the whole quarter. Include non-IFTA miles — this figure is every mile the vehicle ran, not just the taxable ones.
Total gallons
Every gallon placed in the propulsion tank, IFTA jurisdictions and not, rounded to the nearest whole gallon.
Average MPG
Total miles ÷ total gallons, carried to three decimals and then rounded to two. The order matters: rounding early moves the final number.

Worked example, from the form itself

Odometer 0150000 − 0127000 = 23,000 miles. Quarter's receipts total 4,340 gallons. 23,000 ÷ 4,340 = 5.299, which rounds to an average of 5.30 MPG.

Step two

Then one line per jurisdiction, per fuel type

Each jurisdiction you ran in gets its own line, and each fuel type gets its own set of lines. The columns below are lettered the way most states' schedules letter them.

B · Total miles
Miles run in that jurisdiction, from your trip records. These must add up to your total miles from step one — if they don't, something is missing or double-counted.
C · Taxable miles
Usually the same as column B. It differs where an exemption or trip-permit miles apply, and a difference has to be explainable in writing.
D · Taxable gallons
Taxable miles ÷ your average MPG, rounded to a whole gallon. This is the fuel you burned there.
E · Tax-paid gallons
Gallons you actually bought there, from receipts. These must add up to your total gallons and can never exceed them. This is the column receipt capture exists to keep honest.
F · Net taxable gallons
D − E. A negative number here is legitimate — it means you bought more fuel in that jurisdiction than you burned there, and you are owed a credit.
G · Rate
That jurisdiction's rate for that fuel, for that quarter. The current quarter's whole table is published here.
H · Tax or credit
F × G. Negative F gives a credit, which is normal and correct.

The one that is easy to get backwards

A surcharge is not computed like the tax

Indiana, Kentucky and Virginia add a per-gallon surcharge on top of the base rate. It is tempting to add the surcharge to the rate and run one combined number through the same formula. That is wrong, and it understates what you owe.

A surcharge is computed on column D, taxable gallons — the fuel you burned — not on column F, net taxable gallons. It never nets against fuel you bought, and it can never come back as a credit. Combine the two and a carrier who fuels heavily in a surcharge state gets a smaller bill than they owe, or an impossible surcharge refund.

CabPurser shipped this bug once and it was caught against the source text, not by its own tests. That is why the rate table publishes base and surcharge as two separate figures and never as one.

Step three

Add it up, and add interest if you are late

Total
Sum the tax and credit lines across every jurisdiction and fuel type. Credits in one jurisdiction genuinely offset tax owed in another.
Interest
Skipped entirely if you filed by the due date. Otherwise it accrues for each month or fraction of a month the tax is unpaid, separately for every jurisdiction on the return. One day late is a full month's interest.
The interest rate
For a US-based fleet: two percentage points above the IRS underpayment rate (IRC §6621(a)(2)), reset each January 1, accruing at one twelfth of that per month. It is not a fixed number — check the current one rather than reusing last year's.
And the penalty
Separate from interest: $50.00 or 10 percent of the delinquent taxes, whichever is greater — for failing to file, filing late, or underpaying. The percentage side has no cap, and your base jurisdiction may add its own penalties on top.
Zero is still a return
A quarter in which you did not operate or buy fuel anywhere still requires a return. The deadline applies to an empty quarter too.

Penalty and interest are from IFTA's Articles of Agreement R1220.100 and R1230 (rev. 2026-03-11), the interstate agreement itself. Column rules, rounding, the surcharge basis, the worked example and the zero-activity rule are stated in Florida HSMV Form 85800 (rev. 4/16), How to Calculate the IFTA Quarterly Tax Return (flhsmv.gov) — one state's published instructions for the same interstate agreement. Your base jurisdiction publishes its own schedule and its own interest rate; read that one before you file.

This page explains a calculation. It is not tax advice, and following it is not a substitute for a qualified preparer reviewing your return.

Not live yet

This is the part worth automating

None of it is hard. It is just four hours of transcription a quarter, and every step of it is somewhere a wrong number can enter.

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Goes to a person, not a drip sequence.