Intro: Where most business owners go wrong

Most small-business owners know their phone, internet, and home office are part of doing business — but few know how much they can actually deduct, or what proof the IRS expects. The goal isn’t perfection — it’s documentation and consistency. When you can explain your business-use percentage in one sentence and back it with one page of support, you’re audit-ready.

Define what “mixed-use” really means

A mixed-use expense is anything used for both personal and business purposes — your laptop, cell phone, internet, or vehicle.
The IRS doesn’t deny these automatically; it simply requires you to separate the business portion from personal use.

• Personal portion = nondeductible.
• Business portion = deductible as long as it’s ordinary, necessary, and documented.

If it’s used 80% for business, you can deduct 80% — not 100%. That one distinction saves thousands during an audit.

Cell phones & internet: how to document properly

• Keep one month of usage detail (phone bill, call log, or data summary).
• Highlight the business calls, meetings, or uploads.
• Calculate the business-use percentage once per year, apply that ratio monthly, and note “based on January usage study.”
• If you have two phones — one personal, one business — deduct the business line in full and keep the personal one separate.

Pro Tip: Don’t expense your entire family phone plan. Document your line, your share, and your reason for claiming it.

Home office: simplified vs. actual method

Simplified method:

• $5 per square foot, up to 300 sq ft (max $1,500).
• No depreciation tracking.

Actual expense method:

• Calculate the ratio of your office area to total home square footage.
• Apply that ratio to eligible costs — rent, utilities, insurance, repairs.

To qualify, the space must be used regularly and exclusively for business. Occasional use doesn’t count.

Amanda’s test: “If you can’t walk into that space and say this is where my business lives, it’s not a home office.”

Vehicles & travel gear

Many owners lump car, gas, and tolls under “auto expense” without tracking business use. That’s where deductions get lost.

• Keep a mileage log (date, destination, business purpose, miles).
• At year-end, divide business miles by total miles to get your percentage.
• Apply that ratio to gas, insurance, and repairs if using actual expense method, or take the standard mileage rate (70¢ per mile for 2025).
• Commuting to a regular office is never deductible; client visits and supply runs are.

Computers, furniture & office equipment

Mixed-use tech is deductible proportionally.
If your laptop serves business 75% of the time and personal 25%, you can depreciate or expense 75% under the de minimis safe harbor ($2,500 per item limit).
Attach an annual memo stating your basis and percentage — it proves intent and consistency.

The documentation hierarchy (what the IRS actually looks for)

• Proof of payment – receipt, invoice, or credit card record.
• Proof of business purpose – note on invoice or digital annotation (“Client calls; 80% business use”).
• Proof of percentage – mileage log, phone-bill highlight, home-office calculation.
• Consistency across years – same ratio unless circumstances change.

Digitize everything; use folders titled “2025_CellPhone,” “2025_HomeOffice,” etc. That single step keeps your audit trail one click away.

How Pettay Consulting bridges the gap

Most bookkeepers just post the full bill; most CPAs only see totals. I step in between — reviewing line-by-line, recalculating business-use percentages, and coding each item to match IRS guidance. The result: compliant deductions, defensible records, and a P&L that truly reflects how you run your business.

Quick checklist: What to do this week

•  Identify all mixed-use categories (phone, internet, vehicle, home office, tech).
•  Pick your tracking method (simplified vs actual).
•  Capture proof for one representative month.
•  Add a line-item note in QuickBooks for your business percentage.
•  Create a 2025 “Mixed-Use Deductions” folder.

Conclusion: Keep it clear, keep it consistent

Mixed-use expenses are not gray areas when you treat them like business assets with measurable use. You don’t need to overthink — you just need a repeatable system.

Ready to tighten your documentation and boost legitimate deductions?
Book a Deduction & Transaction Review with Pettay Consulting.
Together we’ll quantify your business use, clean up your categories, and make next tax season painless.