If you drive your own car for your business, the mileage deduction can be worth thousands of dollars a year. It is also one of the deductions the tax rules are strictest about. Without a log, the IRS can disallow it entirely, even when nobody doubts that you drove for work.
This is what the IRS asks a mileage log to show, taken from Publication 463 and the regulations behind it, and what it does not ask for.
Why the rules are strict for cars
For most business expenses, courts can accept a reasonable estimate when the records are imperfect. This is known as the Cohan rule. Cars are different. Section 274(d) of the tax code sets strict substantiation rules for listed property, which includes passenger cars, and the Tax Court held in Sanford v. Commissioner (1968) that the Cohan rule does not apply to those expenses. Publication 463 says it in one line: “You can’t deduct amounts that you approximate or estimate.”
That is why the log matters more than the app, spreadsheet or notebook you keep it in.
What the log must show
The regulations list what has to be proved for a car used in business (Treasury Regulation §1.274-5T(b)(6)):
| Element | In a mileage log |
|---|---|
| Amount of each business use | The miles for each business trip |
| Total use | All miles driven in the car during the year, business and personal |
| Time | The date of each business trip |
| Business purpose | Why you made the trip |
Publication 463’s sample daily log uses the columns most people follow in practice: date, destination (city, town or area), business purpose, odometer start and stop, and miles for the trip. The odometer columns are one way to get the miles, not a separate requirement.
Destination and purpose
“Business” or “client meeting” says very little. A line someone can check says who and what: “Site visit, Jones kitchen remodel, 45 Oak Ave.” The sample log only asks for a city, town or area, but an address or a client’s name ties the trip to your calendar and invoices.
Publication 463 allows one shortcut. When the business purpose is clear from the circumstances, you don’t need to write it out. Its example is a sales representative on an established route, who can record the length of the route once, the date of each trip at or near the time, and the total miles for the year.
When to write it down
The standard is a record made at or near the time of the trip. In the regulations’ words, such a record “has a high degree of credibility not present with respect to a statement prepared subsequent thereto when generally there is a lack of accurate recall.”
That does not mean logging every trip the same day. Publication 463: “If you maintain a log on a weekly basis that accounts for use during the week, the log is considered a timely kept record.”
What does not qualify is a log written in April, from memory, for the whole previous year.
Paper, spreadsheet or app
The IRS does not prescribe a format. Publication 463 lists “an account book, diary, log, statement of expense, trip sheets, or similar record”, and says a record prepared on a computer is considered adequate. A notebook in the glove box, a spreadsheet updated every Friday and a GPS tracker app can all work, as long as the elements are there and the entries were made on time.
A tracker app fills in the date and the miles by itself. It cannot know why you drove. Whatever you use, the business purpose is still yours to add, trip by trip.
A sample instead of a full year
If your driving follows a steady pattern, Publication 463 allows sampling: keep an adequate record for part of the year and use it to prove business use for the whole year, as long as other evidence shows the sampled periods are representative. Its example is a full log for the first week of each month showing 75% business use, with invoices and bills showing the same pattern for the rest of each month.
Sampling suits a steady routine. If your driving changes from week to week, a full log is easier to defend.
Two rates in 2026
The IRS changed the business standard mileage rate partway through 2026: 72.5 cents a mile from January 1, and 76 cents from July 1. If you use the standard mileage rate, you need your 2026 business miles split into before and after July 1. A log with a date on every trip already gives you that.
What the tax return asks
If you claim car expenses on Schedule C and use the standard mileage rate, Part IV asks for the miles you drove during the year for business, for commuting and for other purposes. It then asks two questions: do you have evidence to support the deduction, and is that evidence written? (Some filers answer the same questions on Form 4562 instead.) The log is what lets you answer yes to both.
Commuting is asked for separately because commuting miles are not business miles, however work-related they feel.
How long to keep it
Generally three years from the date you file the return that claims the deduction, according to Publication 463. If you deduct actual expenses and depreciate the car, keep records of its business use for each year of the recovery period.
Checklist
- For every business trip: date, destination, business purpose and miles.
- Total miles for the year. Odometer readings at the start and end of the year are the simplest way.
- Entries made the same day, or at least weekly.
- Purposes specific enough to match against your calendar or invoices.
- For 2026, business miles before and after July 1.
- The log kept for at least three years after you file.
Why I am writing about this
I am building Milestamp, a mileage log for self-employed people in the US who drive their own car for work. It is not released — no App Store listing, no release date — and the app page says what it is being built for rather than promising features. The rules above are the rules it has to satisfy, which is why they are written out here in full.
Short answers
What does the IRS require in a mileage log? For each business trip, the date, the miles and the business purpose, plus the total miles driven during the year. Most logs also record the destination.
What is a contemporaneous mileage log? A log written at or near the time of the trips. A log kept weekly counts.
Do I need a mileage log to deduct business mileage? Yes. The IRS does not allow estimated amounts for car expenses.
Is a mileage tracker app IRS compliant? The IRS does not approve apps. A log from an app is adequate if it has the required details and was kept at the time, like any other record.
This is general information about IRS rules, not tax advice. For your own situation, ask a tax professional.