The IRS mileage log requirements come from Section 274(d) of the Internal Revenue Code, and they are strict: for every business trip you claim, your records must show the date, the miles driven, the destination, and the business purpose of the trip. Miss any of those four elements and the deduction can be disallowed entirely. Vehicle expenses fall under a heightened substantiation standard that does not allow estimates or reconstructions from memory, so the log you keep during the year is the document your entire deduction rests on.
The Four Elements Every Entry Must Contain
Publication 463 translates the statute into four pieces of information that belong in every log entry.1Internal Revenue Service. Publication 463 – Travel, Gift, and Car Expenses
- Date. The specific date of each trip. A range like “every Tuesday in March” will not hold up.
- Mileage. The distance driven for each business use, plus the total miles driven for the year across business, commuting, and personal use combined. Odometer readings at the start and end of each trip provide the strongest evidence.
- Destination. Where you went, with enough specificity that an auditor can verify it. “Client meeting” is too vague. “Anderson Architecture, 450 Main St, Springfield” is not.
- Business purpose. Why the trip was necessary for your work. “Delivered project blueprints to client for final review” is sufficient. “Business” or “work” by itself is not, and auditors disallow those entries almost automatically.
The purpose line should make it obvious to a stranger why the trip generated income or was necessary for your trade or business. Vague single-word entries are the most common reason logs fail on audit.
Record Entries at or Near the Time of the Trip
The regulation governing vehicle substantiation requires each entry to be made “at or near the time of the expenditure or use.”2eCFR. 26 CFR 1.274-5T – Substantiation Requirements (Temporary) Publication 463 puts it plainly: “A timely kept record has more value than a statement prepared later when there is generally a lack of accurate recall.”1Internal Revenue Service. Publication 463 – Travel, Gift, and Car Expenses
Daily entries are the best practice. Weekly is still defensible. A log reconstructed from bank statements and calendar invites at tax time is exactly the kind of record that gets thrown out.
Paper Logbooks and Digital Apps Both Qualify
The IRS accepts handwritten logbooks and digital records alike, as long as the record is permanent and not easily altered after the fact. A computer-based log maintained with the aid of a logging program qualifies as an adequate record under the regulations.2eCFR. 26 CFR 1.274-5T – Substantiation Requirements (Temporary)
Mileage tracking apps that use GPS to capture distances and locations automatically produce timestamped entries that are hard to dispute. If you use one, confirm it allows data export. Some apps delete trip history when you cancel your subscription, which defeats the point of keeping records for years afterward. The subscription cost itself is deductible as a business expense.
The Sampling Method for Partial-Year Logs
You do not necessarily have to log every trip for all twelve months. The IRS allows detailed records for a representative portion of the year to establish a pattern that is then applied to the full year, provided the sample fairly reflects your driving. Publication 463 gives the example of tracking the first week of each month.1Internal Revenue Service. Publication 463 – Travel, Gift, and Car Expenses
You still need odometer readings at January 1 and December 31 to establish total annual mileage. Sampling works poorly if your driving changes seasonally or your work situation shifted during the year. In those cases, a full-year log is safer.
Which Miles Belong in the Log
Only deductible business miles count toward your deduction, and the boundaries matter for what you record. Driving from your home to your regular workplace is commuting, and commuting is never deductible under Section 162, even if you take business calls on the way or your vehicle carries your company’s branding.3Office of the Law Revision Counsel. 26 USC 162 – Trade or Business Expenses
Trips that do belong in the log include travel between your regular office and a client’s location, driving between two client sites, and trips from your regular workplace to a temporary work location. A work location is “temporary” when it is realistically expected to last one year or less.1Internal Revenue Service. Publication 463 – Travel, Gift, and Car Expenses
The Home Office Exception
If you have a home office that qualifies as your principal place of business, the commuting rule flips. Travel from your home to any other work location in the same trade or business becomes deductible regardless of distance.4Internal Revenue Service. Publication 587 – Business Use of Your Home To qualify, the space must be used exclusively and regularly as your principal place of business. The IRS looks at both the relative importance of the work performed at each location and the time spent there.1Internal Revenue Service. Publication 463 – Travel, Gift, and Car Expenses
What Happens If Your Log Is Inadequate
Vehicle expenses are one of the few areas where the IRS gives no slack on documentation. Under Section 274(d), if you cannot substantiate the required elements with adequate records, the deduction is disallowed entirely. Not reduced, not estimated, but gone.5Office of the Law Revision Counsel. 26 USC 274 – Disallowance of Certain Entertainment, Etc., Expenses
For most other deductions, the Cohan rule lets courts estimate a reasonable amount when records are incomplete. That escape hatch does not exist for vehicle expenses. Courts have consistently held that the Cohan rule does not apply to items subject to the strict substantiation requirements of Section 274(d).
On top of losing the deduction, the resulting underpayment can trigger an accuracy-related penalty of 20% of the underpaid tax. The IRS imposes this penalty when the understatement is due to negligence, defined as failing to make a reasonable attempt to follow tax law, or when it constitutes a “substantial understatement,” which for individuals means the understated amount exceeds the greater of 10% of the correct tax or $5,000.6Internal Revenue Service. Accuracy-Related Penalty A $10,000 mileage deduction you cannot substantiate can cost you the deduction, the back taxes, and 20% on top.
Standard Mileage Rate and Actual Expenses Both Require the Log
The choice between the standard mileage rate and the actual expense method does not change what the log has to contain. Both methods require the same four elements per trip. The actual expense method simply adds a second layer: receipts for fuel, oil, insurance, tires, repairs, registration, lease payments, parking, and tolls, so you can apply your business-use percentage to total vehicle costs.
Your log produces that percentage. Divide business miles by total miles driven for the year. If you drove 20,000 miles total and 14,000 were for business, your business-use percentage is 70%. Under the 2026 business standard rate of 72.5 cents per mile, 14,000 business miles is a $10,150 deduction.7Internal Revenue Service. 2026 Standard Mileage Rates (Notice 2026-10)
Getting that percentage right matters beyond a single year’s deduction. Claiming accelerated depreciation or bonus depreciation on a vehicle requires business use above 50%, and the log is the only way to prove you cross the line.8Office of the Law Revision Counsel. 26 USC 280F – Limitation on Depreciation for Luxury Automobiles
How Long to Keep the Log
Keep your mileage log and supporting receipts for at least three years after filing the return that claims the deduction. That matches the general statute of limitations for the IRS to assess additional tax.9Internal Revenue Service. How Long Should I Keep Records If you underreported gross income by more than 25%, the IRS has six years, so holding records that long is reasonable if there is any uncertainty. The three-year clock runs from the filing date or the due date, whichever is later; returns filed early are treated as filed on the due date.10Internal Revenue Service. Publication 583 – Starting a Business and Keeping Records
Store digital copies alongside paper originals. A cloud backup protects against a fire, a flood, or a hard drive failure wiping out years of documentation at once.
A Note for W-2 Employees
Most W-2 employees currently cannot deduct unreimbursed vehicle expenses at all, regardless of how meticulous their log is. The Tax Cuts and Jobs Act suspended the miscellaneous itemized deduction for unreimbursed employee expenses for tax years 2018 through 2025. The only employees who can use Form 2106 during the suspension are Armed Forces reservists, qualified performing artists, fee-basis state or local government officials, and employees with impairment-related work expenses.11Internal Revenue Service. Instructions for Form 2106 Whether the suspension actually lifts for 2026 depends on recent legislation, so confirm with a tax professional before claiming unreimbursed employee vehicle expenses on a 2026 return.