Is a vehicle logbook legally mandatory?

Neither administration words it as an absolute obligation for your own vehicle. The Canada Revenue Agency says the best evidence to support the use of a vehicle is an accurate logbook of business travel maintained for the entire year, and Revenu Quebec says that to support your calculation you may keep a vehicle use register. What is not optional is the result: the burden of proving the business share is yours. Without a record the percentage is an estimate, and an estimate is the thing that gets reduced on review.

I did not keep a log this year. What should I do now?

Start one today, because a partial record is worth more than none, and the months still ahead of you are the ones you can still document properly. For the months already gone, rebuild only from things that genuinely exist: calendar entries, client appointments, invoices showing an address, delivery or job records. Do not invent round numbers, because a clean estimate with nothing underneath it is exactly what an auditor is trained to notice. Then keep a full twelve-month log next year, which also gives you a base year so later years can run on a three-month sample.

What does the CRA want in a vehicle logbook?

The deduction is a ratio, not a receipt

Most vehicle expenses are not deducted as amounts. They are deducted as a share. If you use a vehicle for both business and personal driving, you deduct only the part you paid to earn income, and that part is set by kilometres: business kilometres divided by total kilometres, applied to the year's vehicle costs.

The CRA's own example makes the mechanic plain. Danielle drives 27,000 business kilometres out of 30,000 total, and her van costs $5,400 for the year in licence and registration, gas and oil, insurance, interest, and maintenance. Her deduction is 27,000 divided by 30,000, multiplied by $5,400, which is $4,860.

Notice what carries the whole calculation. Every receipt in that pile proves an amount was spent. Not one of them proves the trip was for business. Only the log does that.

What goes in it

For each business trip, record four things: the date, the destination, the purpose, and the number of kilometres.

Then two more, at the level of the year itself. Record the odometer reading of each vehicle at the start and the end of the fiscal period, because the business number means nothing without the total it sits over. And if you change vehicles mid-year, record the date of the change and the odometer reading when you buy, sell, or trade.

If more than one vehicle is used in the business, keep a separate record for each, showing both its kilometres and its running costs. The vehicles are calculated separately, not pooled.

The three-month shortcut

A full year of logging is the standard, and the CRA offers a way out of repeating it forever.

Keep a full logbook for one complete year to establish a base year. In later years you can keep a logbook for only a three-month sample period and project the year from it, as long as usage stays in the same range. The formula is the sample period percentage divided by the base year's percentage for those same months, multiplied by the base year's annual percentage.

The CRA's illustration: a base year at 49% annual business use, with 46% across April to June. A later year's April-to-June sample shows 51%. So 51 divided by 46, times 49, gives 54% for that year, and the CRA accepts it because 54 sits inside the 10 point band around 49, meaning between 39% and 59%.

Fall outside that band and the base year is no longer representative. The sample is then good only for the three months you actually logged, and you should start a new base year.

One retention detail people miss: that base year logbook has to be kept six years from the end of the last tax year it was used to establish business use, not six years from the year you wrote it.

What the percentage applies to

Your business-use share applies to licence and registration fees, fuel and oil, electricity for a zero-emission vehicle, insurance, interest on money borrowed to buy the vehicle, maintenance and repairs, and leasing costs. Those go on line 9281 of form T2125. Capital cost allowance is claimed too, but on line 9936.

Two items are not prorated. Business parking fees and supplementary business insurance are deductible in full, because they are business costs outright rather than a slice of a shared expense.

If the vehicle is a passenger vehicle, ceilings apply on top of your percentage. For 2026, the capital cost allowance ceiling for a Class 10.1 passenger vehicle is $39,000 before tax, up from $38,000, for vehicles acquired on or after January 1, 2026. Deductible leasing costs stay at $1,100 a month before tax, and the maximum interest deduction stays at $350 a month, both for new leases and loans entered into on or after that date. For a Class 54 zero-emission passenger vehicle the ceiling remains $61,000 before tax.

There is no flat per-kilometre option

You may have seen the 2026 rates of 73 cents per kilometre for the first 5,000 kilometres and 67 cents after that. Those are not a write-off you can claim for your own vehicle. They are the ceiling on the tax-free allowance an employer can pay an employee who drives their own car for work.

A self-employed person does not get a simple rate. You claim actual costs, and you prorate them. Which brings it back to the same place: the percentage is the deduction, and the log is the percentage.

A note in your phone after each appointment is enough. Date, where, why, how far.