What Happens If I Estimate My Mileage Instead of Keeping an Exact Log?
Tax season rolls around, and someone who drove for deliveries or client visits all year tries to reconstruct their mileage from memory. A rough estimate feels close enough. Whether it actually holds up is a different question.
The short answer
A rough estimate of business mileage generally does not meet the recordkeeping standard expected for a mileage deduction, which typically calls for a contemporaneous record showing the date, destination, business purpose, and miles driven for each trip. Estimates can be challenged or disallowed if the deduction is ever reviewed, since there’s no documentation tying the number to actual activity. Many people rely on a mileage log or tracking app precisely because reconstructing a year of driving from memory afterward is difficult to defend.
Why “close enough” creates real risk
- There’s no paper trail to point to. If a deduction is questioned, an estimate with no supporting log leaves little to show beyond a person’s own recollection, which generally isn’t sufficient documentation on its own.
- Estimates tend to drift high. People often round up when remembering trips after the fact, and that gap between remembered and actual mileage is exactly what a documentation requirement exists to catch.
- Personal and business driving get blurred. Without records made close to the time of each trip, it becomes harder to separate legitimate business mileage from commuting or personal errands, which generally aren’t deductible.
- The deduction itself may still be valid — the support for it just isn’t. Someone may have genuinely driven for business purposes all year, but if the records don’t exist, proving it after the fact is the actual problem.
What a contemporaneous log generally includes
A typical mileage log records the date of each trip, the starting and ending locations or purpose, and the number of miles driven, ideally noted at or near the time of the trip rather than reconstructed later. Many people use a dedicated app or a simple notebook kept in the vehicle. This kind of documentation matters most for people newly earning income this way, and it pairs with other recordkeeping habits worth knowing, including how long tax records should generally be kept once a return has been filed.
What happens if a return is filed using rough estimates
Filing with an estimated number doesn’t necessarily trigger an immediate problem, but if a return is later reviewed, an estimate without supporting records is a common point of disallowance. This can mean owing back the value of the deduction, plus interest, and possibly penalties depending on the circumstances. It’s part of why understanding what happens if you file your taxes late and other recordkeeping consequences matters for anyone earning income outside a traditional paycheck, where documentation habits aren’t already built into payroll withholding. This kind of gap often surfaces alongside other first-year surprises, like realizing that side income doesn’t typically have any taxes withheld from it at all, which makes building a documentation habit early even more worthwhile.
Rebuilding a log after the fact
Someone who didn’t track mileage in real time isn’t necessarily out of options, but reconstructing records after the year has ended generally requires more supporting evidence — receipts, calendar entries, delivery app history — to make a reconstructed log credible. This is more work than logging trips as they happen, which is part of why the habit is worth building early rather than trying to recreate it under pressure later.
The takeaway
Estimating mileage instead of logging it in real time creates a documentation gap that can undermine an otherwise legitimate deduction. Building a simple, consistent logging habit from the start is generally far easier than trying to reconstruct a defensible record after the fact.