Business vs Personal Expense Allocator
When you use something for both work and life — your phone, car, or internet — only the business share is deductible. This splits a mixed-use expense into its deductible and personal parts.
Your details
Rough figures are fine — you can refine later.
How we calculate this
We isolate the deductible share of a mixed-use expense.
1. Deductible portion. Total expense × business-use %. Only the part attributable to business is deductible on Schedule C.
2. Personal portion. The remainder — not deductible.
The business-use percentage should come from real records (a log, representative statements), not an estimate. Some expense categories carry additional limits (for example, meals are generally 50% deductible; vehicles follow separate rules).
This is an estimate, not tax advice, and is queued for CPA/EA review. Confirm both your percentage and any category-specific rules with a tax professional.
Primary sources
- IRS Publication 535, Business Expenses (ordinary and necessary; business-use allocation)
- IRS Schedule C (Form 1040) Instructions
Only the business share is deductible
Some of your biggest expenses do double duty. Your phone is for clients and your family. Your car runs to job sites and to the grocery store. Your home internet powers work and Netflix alike. For expenses like these, the tax rule is simple in principle: you can deduct only the portion you use for business.
This calculator does that split. Enter the full cost of a mixed-use expense and the percentage you use it for work, and it separates the deductible business portion from the personal part you can’t write off.
How the split works
The math is a single multiplication: total expense × business-use percentage. A $1,200 annual phone bill used 60% for business yields a $720 deduction and $480 of personal cost. Do this for each mixed-use expense and you capture every legitimate dollar without overreaching into personal spending that doesn’t qualify.
The number that matters most is the percentage — and it has to be real. A business-use share should come from actual records: a mileage log for a vehicle, a few representative months of itemized phone use, a usage log for equipment. The IRS expects a reasonable, supportable basis, and a suspiciously round “100% business” on something you obviously also use personally is a classic audit flag.
Common mixed-use expenses
The usual suspects are cell phone and internet, a personal vehicle used for business travel, a laptop or camera, software you use for both work and personal projects, and utilities tied to a home office. Each should be deducted only to the extent it serves the business.
A few categories carry extra rules on top of the business-vs-personal split. Business meals are generally limited to 50% deductible. Vehicles can be deducted using either actual costs or the standard mileage rate, but not a casual mix. Treat this calculator’s result as the deductible business share, then apply any category-specific limit — our mileage and home-office tools handle two of the big ones.
Where it connects
This allocator feeds naturally into the write-off estimator, which totals your deductions into a tax saving, and sits alongside the home-office and mileage calculators, which are really just specialized versions of the same business-use idea applied to a home and a car.
What this is
An estimate, not tax advice, and it’s queued for CPA/EA review before publish. It handles the business-vs-personal allocation — the first and most common step. Confirm your usage percentage and any category-specific rules with a tax professional, and keep the records that back up your numbers.
Common questions
How do I split a mixed-use expense? + −
Multiply the total cost by the percentage you use it for business. If your phone bill is $1,200 a year and you use it 60% for work, $720 is deductible and $480 is personal. The key is that the percentage must reflect actual, documented use — not a convenient round number.
What expenses are commonly mixed-use? + −
Cell phone and internet, a personal vehicle used for client visits, a laptop, a home utility bill tied to a home office, and software you use for both work and personal projects. Each should be deducted only to the extent it’s used for business.
How do I prove my business-use percentage? + −
Keep contemporaneous records: a mileage log for a car, a few representative months of itemized phone use, or a usage log for equipment. The IRS expects a reasonable, supportable basis for the percentage. Guessing high without records is a common audit trigger.
Is the business portion always fully deductible? + −
This tool handles the business-vs-personal split, which is the first step. Some categories have extra rules — meals are generally 50% deductible, and vehicles can use either actual costs or the standard mileage rate. Treat the result as the deductible business share, then apply any category-specific limit and confirm with your tax pro.
Keep going
Prepared for tax year 2026. Every rate and cap on this page cites a primary IRS or SSA source. Estimates only — not tax or financial advice. — for planning purposes only, not tax, legal, or financial advice.