What the IRS actually requires in a mileage log. It is four things, and all four.

Date, mileage, destination, business purpose. Under section 274(d) substantiation is all or nothing, and three out of four earns you nothing.

ExactSplit · Updated March 10, 2026

Vehicles are held to a higher standard

Most business expenses are deductible if you can show, with reasonable records, that you paid them and that they were ordinary and necessary. A receipt and a bank statement usually do the job. Vehicle expenses are different, and the difference is written into the code.

Section 274(d) singles out a few categories, including travel, gifts, and listed property, and says no deduction is allowed for them unless the taxpayer substantiates specific elements by adequate records or by sufficient evidence corroborating the taxpayer's own statement. Passenger vehicles are listed property under section 280F. So a mileage deduction has to meet the stricter 274(d) standard, not the general one.

The four elements

Publication 463 lays out what has to be substantiated for vehicle use, and it comes down to four things for each business trip.

The first is the date. When did the trip happen. The second is the mileage, either the distance driven or the odometer readings at the start and end. The third is the destination, where you went. The fourth is the business purpose, why the trip was business rather than personal.

The business purpose does not need to be elaborate. Client meeting at Northwind Labs is a business purpose. Site visit, 400 Market Street is a business purpose. Business, on its own, is not. Errands is not. The purpose has to connect the trip to the trade in a way someone else could verify.

Why three out of four is zero

The most important thing to understand about 274(d) is that it is not a scoring system. A trip with a date, mileage, and destination but no business purpose is not a mostly substantiated trip. It is an unsubstantiated trip, and on examination the deduction for it is disallowed in full.

This is where mileage differs from almost every other deduction. For a normal expense, weak records might get you a reduced deduction under the Cohan rule, which lets a court estimate an amount when the expense clearly happened but the records are imperfect. Section 274(d) specifically overrides Cohan for the categories it covers. There is no estimation. Either the elements are substantiated or the deduction is gone.

Software that treats the four fields as optional is therefore doing something worse than being lax. It is letting you build a log full of trips that cannot be deducted, while displaying a deduction total as if they could.

Contemporaneous versus reconstructed

The regulations under 274(d) say adequate records means a log, diary, or similar record made at or near the time of the expense. A log you fill in each week is contemporaneous. A spreadsheet you rebuild from your calendar in March, with mileage estimated from a mapping app, is a reconstruction.

Reconstructed logs are not automatically rejected, but they carry much less weight. Tax Court cases on mileage frequently turn on exactly this point. A log with the same handwriting and pen for the whole year, or a spreadsheet created the week before the audit, is treated with suspicion. A log with entries that vary in detail and are clearly made over time is treated as credible.

The practical advice is to log trips when they happen, or at least within the same week. It takes thirty seconds and it is the difference between a deduction that survives and one that does not.

Commuting is never deductible

One category of driving fails substantiation not because of missing records but because it is not business mileage at all. Driving from home to your regular place of business and back is commuting, and commuting is personal under section 262 regardless of how far it is or how much work you do while driving.

For a self-employed person with a home office that qualifies as a principal place of business, this changes. Trips from the home office to a client site are business trips, because the home office is the regular place of business and the client site is a temporary one. If the home office does not qualify, the first trip of the day from home to a work location is commuting and only trips between work locations count.

Whether your home office qualifies is its own question with its own rules under section 280A. It is worth getting right, because it determines whether your first and last trips of every day are deductible or not.

The rate already includes the car

A related error is worth mentioning because it shows up in almost every bank feed. The standard mileage rate covers fuel, oil, maintenance, repairs, insurance, registration, and depreciation. If you take the rate, all of those costs are already deducted.

So a gas station charge in your transactions is a business expense in the ordinary sense, but it is not separately deductible if you also claim mileage. Deducting both is claiming the same cost twice. The only vehicle costs that stack on top of the mileage rate are parking and tolls incurred on business trips.

If you would rather deduct actual costs, you can, but then you do not take the mileage rate and you have to track the business percentage of every vehicle expense. It is one method or the other per vehicle, and the choice has to be made in the first year the vehicle is used for business if you want the standard rate to be available later.

Odometer readings and the annual total

In addition to per trip records, you need the total miles the vehicle was driven for the year, business and personal combined. Schedule C asks for this directly in Part IV. The simplest way to have it is to record the odometer on January 1 and December 31.

The business use percentage, which is business miles divided by total miles, matters for actual expense method users directly and is a reasonableness check for everyone. A log showing 30,000 business miles on a vehicle that was driven 32,000 miles total is going to draw attention if the owner also uses it to get groceries.

How ExactSplit handles it

ExactSplit's trip entry requires all four elements. The form will not save a trip without a date, a distance, a destination, and a business purpose, and the database will not store one, because a trip that is missing any of them is not a deductible trip and there is no point pretending otherwise.

Each trip is priced by its own date, which matters in 2026 because the rate changed at midyear. The Schedule C export includes the full log with all four fields and the rate applied to each trip, so a preparer has the substantiation in hand rather than a total they have to take on faith. And if fuel or repair transactions are also marked deductible, the mileage section says so, because that is the double count.