A mixed-use expense is deductible for the business portion and never for the personal portion. The paper trail matters as much as the expense: the IRS requires strict substantiation (amount, time, place, business purpose) for travel, meals, vehicles, and gifts. Document contemporaneously and keep business money separate.

What counts as a mixed-use expense?

Any expense that serves both business and personal purposes: the cell phone you use for client calls and family texts, the car you drive to job sites and to the grocery store, the trip that combined a client meeting with a long weekend, the home internet that runs both the business and the streaming. The tax treatment is always the same: you deduct the business portion, and you prove the split with records.

Two categories live outside this framework entirely. Pure business expenses (the office lease, the business-only software) need only show they are ordinary and necessary. Pure personal expenses (the family vacation, the daily wardrobe) are never deductible under IRC 262, no matter how you label them. Mixed-use is the middle ground, and it is where most deduction disputes happen.

What does the IRS require you to prove?

For most business expenses, adequate records are enough: receipts plus a credible business purpose. But IRC 274 imposes a stricter substantiation standard on four categories: travel, entertainment (now largely nondeductible), gifts, and listed property (including vehicles). For these, you must document each element, at or near the time of the expense:

Reconstructed logs created months later, at tax time, carry little weight. The IRS wants records made when the memory is fresh. A contemporaneous note in your phone ("dinner with Patel re: Q3 scope, $84, Austin") beats a perfect spreadsheet built in April. (IRS Publication 463)

How do you document each category?

Vehicle. Choose the standard mileage rate or actual expenses at the start, then log every business trip: date, destination, miles, business purpose. For 2026 the rate is 72.5 cents per mile for January through June and 76 cents per mile for July through December. Commuting from home to your regular workplace is never deductible; travel between business locations during the day is. (IRS, Standard Mileage Rates)

Travel and meals. Keep receipts for lodging and for any expense of $75 or more, and record the business purpose of the trip. Business meals are 50% deductible with proper substantiation of amount, time, place, and business purpose. Entertainment is not deductible at all, so the client dinner after the game is a meal (document the business discussion) while the game tickets themselves are not deductible.

Phone and internet. Deduct the business-use percentage. If half your phone usage is business, deduct half the bill. The percentage has to be real: "100% business" on a phone your kids also use will not survive five minutes of questioning. A reasonable, documented estimate based on actual usage patterns is defensible; a round number pulled from the air is not.

Dues and memberships. Professional dues and subscriptions directly related to your business are deductible. Illustrative example: say you pay $8,000 a year for a real estate investor group membership where you source deals and get market analysis. If the membership is ordinary and necessary for your business, it is deductible; if it is mostly a social club with a thin business veneer, it is not. The test is the same ordinary-and-necessary standard from IRC 162, applied honestly. (IRS Publication 334)

What is categorically personal?

Some expenses feel business-adjacent but are personal as a matter of law:

• Volunteer travel. Driving for a nonprofit sports team or charity event is not a business expense. You can deduct unreimbursed out-of-pocket costs as a charitable contribution (mileage at the fixed 14-cent charitable rate), but only if you itemize, and the value of your time is never deductible. (IRS Publication 526)

• Volunteer travel with no charitable or business connection. Travel for a cause that is neither a qualified charity nor your business is personal. It is not a write-off because you wish it were.

• Family expenses. Childcare so you can work, the family car, the household groceries: personal under IRC 262, even though they obviously support your ability to run the business.

What does a defensible paper trail look like?

Illustrative example: say you run $20,000 of business spend through a personal credit card during the year. The expenses themselves may be perfectly legitimate, but proving it means untangling every charge from personal spending, producing a business purpose for each one, and hoping the card statements survive as adequate records. The deduction is theoretically available and practically painful.

Now compare the same $20,000 run through a dedicated business card with a simple system:

1. Separate accounts. Business expenses on the business card, personal on the personal card. Separation is the cheapest audit insurance you can buy.

2. Receipts captured at purchase. A photo of the receipt, saved the same day, with the business purpose in the filename or notes.

3. Mileage logged per trip. Date, destination, miles, purpose, recorded the same day.

4. Monthly reconciliation. Thirty minutes a month matching receipts to statements, while the purposes are still fresh.

The same expenses, documented this way, are boring. Boring is exactly what you want a deduction to be.

Updated October 28, 2026.

This article is free educational content, not tax advice for your specific situation. If you want a plan built around your actual numbers, that is what our paid tax planning engagements do.