If you drive for work, your mileage log is the single document standing between you and a legitimate tax deduction. Without it, the IRS can disallow your vehicle write-off entirely — even when the business use is real and obvious. The good news is that a complete log isn't complicated; it just has to capture the right details, consistently, at the right time. Here's exactly what belongs in one and how to keep it audit-proof.
Why a Mileage Log Matters
The IRS requires contemporaneous records to support any business vehicle deduction. A business owner without a mileage log generally cannot claim vehicle expenses at all, regardless of how much the car is genuinely used for work. In other words, the deduction lives and dies by the documentation.
This matters more than most drivers realize. If you use the standard mileage rate — 67 cents per mile for 2026 — every mile you can prove is worth real money. Someone driving 25,000 business miles deducts $16,750 (25,000 × $0.67). That figure is only defensible if your log backs it up trip by trip.
The Five Elements Every Log Entry Needs
A valid mileage log isn't a vague summary at year's end. Each business trip should be recorded with the same core details so the entry stands on its own if questioned. At minimum, capture:
- Date of travel — when the trip took place.
- Destination — where you drove (the specific location, not just "client").
- Business purpose — the reason for the trip, such as a client meeting, site visit, or supply run.
- Miles driven — the distance for that specific trip.
- Odometer readings — the start and end readings that support the mileage claimed.
Odometer readings deserve special attention. Recording your odometer at the start and end of the year, plus at each trip where practical, lets you reconcile total miles and demonstrate the split between business and personal use. That total-miles context is what makes individual entries credible.
Contemporaneous Means Now, Not Later
The most overlooked requirement is timing. Logs must be kept contemporaneously — created during the period the vehicle is used, not reconstructed from memory months afterward. An end-of-year reconstruction, cobbled together from calendar guesses and rough estimates, may not satisfy the IRS, especially if you can't credibly explain how you recreated months of detailed data.
The practical takeaway: log daily or weekly, not annually. A quick habit of recording each business trip as it happens produces a far stronger record than a frantic December scramble. If you find yourself trying to remember where you drove last spring, you've already fallen behind.
Business Miles vs. Personal Miles
A complete log also has to separate what counts from what doesn't. The classic trap is commuting. Driving from home to a regular office is personal and non-deductible — no matter how far it is. Business travel from that office to a client site, however, is deductible.
Consider a common day: you drive 15 miles from home to the office (non-deductible commuting), then 30 miles from the office to a client (deductible business travel). Only the 30 miles can be claimed. Your log should make this distinction obvious so that personal commuting never quietly inflates your deduction. Clearly labeling the purpose of each leg is what keeps the two categories clean.
Paper, Spreadsheet, or App?
The IRS accepts paper logs — a notebook in the glovebox works perfectly well if you actually fill it in. But manual tracking is where good intentions go to die, which is why many drivers turn to digital tools that timestamp and geolocate trips automatically. Apps such as MileIQ, Everlance, and TripLog can capture the mileage details in the background so you're not relying on memory.
Automated tracking also pairs naturally with the rest of your vehicle recordkeeping; Rienly, for instance, helps you log mileage while keeping your maintenance schedule in view, so the same car data supports both your deductions and your upkeep. Whatever method you choose, the standard is the same: reasonable completeness. Missing the occasional trip may be acceptable, but systematic gaps signal inadequate documentation and invite trouble.
How the Standard Mileage Rate Works
If you use the standard mileage rate, the math is straightforward: multiply your total business miles by the current rate. At 67 cents per mile for 2026, that rate is designed to cover all of your vehicle's operating costs on a per-mile basis — fuel, maintenance, repairs, insurance, and depreciation are all baked in. You don't deduct those separately under this method.
That's precisely why the log is so important with the standard rate: your entire deduction is a function of documented miles. No miles, no deduction.
The Actual Expense Method Alternative
There's a second path. Under the actual expense method, you track and deduct the real costs of operating the vehicle — fuel, maintenance, repairs, insurance, and depreciation — rather than a flat per-mile figure. You then apply the business-use percentage of the vehicle to those costs.
Even here, mileage tracking doesn't disappear. To calculate the business-use percentage, you still need to know your total miles and your business miles for the year. So whichever method you choose, a clean log of business versus total mileage remains the foundation. The difference is that the actual expense method adds a layer of receipt-keeping for every operating cost on top of the mileage record.
Common Mistakes That Undermine a Log
- Reconstructing after the fact. Rebuilding a year of trips from memory rarely holds up.
- Vague purposes. "Business" alone doesn't explain anything; name the meeting, client, or task.
- Claiming commutes. Home-to-office miles are personal, and mixing them in weakens the whole log.
- Ignoring odometer readings. Without start-and-end totals, your trip miles have no anchor.
- Systematic gaps. A few forgotten trips are human; entire missing months look like an estimate.
Putting It All Together
A complete mileage log is really a simple discipline made consistent. Record every business trip while it's fresh, capturing the date, destination, purpose, miles, and odometer readings. Keep personal commuting clearly separated from deductible business travel. And decide early whether you'll use the standard mileage rate or the actual expense method, because both rely on the same underlying trip data.
Done properly, your log turns a legitimate business cost into a defensible deduction — and gives you the paperwork to stand behind it if the IRS ever asks. The best time to start a proper log isn't at tax time; it's the next time you turn the key.