Tax rule US
Business use: the mileage log
The record the IRS accepts for the business share of a vehicle, and the year Bilbx's Tax Lab applies it for.
The record for a vehicle
Mileage log, for its own year
Checked against the IRS's own pages on . This is the figure Bilbx uses.
The rule
You can keep an adequate record for parts of a tax year and use it to prove the business use for the entire year, when those parts represent the use throughout the year.
What the IRS says
“You can keep an adequate record for parts of a tax year and use that record to prove the amount of business or investment use for the entire year.”
“You must demonstrate by other evidence that the periods for which an adequate record is kept are representative of the use throughout the tax year.”
How Bilbx applies it
- Tax Lab, part of Bilbx Premium, applies a category's business-use share for the tax years its record covers: mileage log, for its own year; usage record, for its own year; floor area, until you change it.
- When a record no longer covers the year, Settings asks for a new one.
Worked examples
Bilbx's own tests run these exact cases against the website and the app.
| Record | Made in the tax year starting | Read in | Tax Lab |
|---|---|---|---|
| Mileage log | 2025 | 2025 | Applies the share |
| Mileage log | 2025 | 2026 | Asks for a new record |
| Floor area | 2020 | 2030 | Applies the share |
Sources
Spotted something that doesn't match the IRS's guidance?
Every sentence in the rule is checked against the IRS's own guidance, quoted beneath it and linked above. Last checked . Rules version 60f9162250af. Terms.
More for the US: every tax rule Bilbx applies and guides on receipts and tax records.