Business Expense Allocator
Split a mixed-use expense — phone, car, home internet — into its deductible business share and its personal, non-deductible share.
Written by Dorothy Ibrahim, 10+ years in banking & finance
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How we calculate this
Some of your biggest costs do double duty — your phone, your car, your home internet — serving both work and personal life. This calculator splits a mixed-use expense into the share that is deductible as a business cost and the share that is personal and cannot be written off. Enter the full cost of the item and the percentage of time or use that goes toward business, and the split is calculated instantly.
The formulas
- Business-use fraction
- business-use % ÷ 100, clamped to 0–100%
- Deductible (business) portion
- expense amount × business-use fraction
- Personal (non-deductible) portion
- expense amount − deductible portion
Worked example
- With the defaults — a $1,200 annual expense used 60% for business — the business-use fraction is 60 ÷ 100 = 0.60.
- The deductible portion is $1,200 × 0.60 = $720.
- The personal portion is the remainder: $1,200 − $720 = $480.
- Only the $720 business share reduces taxable business income on Schedule C; the $480 personal share is not deductible.
Rates, benchmarks & sources
- A cost must be ordinary and necessary for the business; for mixed-use property or services, only the business-use share is deductible. — IRS Publication 535, Business Expenses
- Business expenses reported on Schedule C must reflect the business-use portion of any mixed personal/business item. — IRS Schedule C (Form 1040) Instructions
Figures current as of 2026-07-02. See our methodology & editorial standards for how constants are versioned and verified.
What this tool doesn’t model
- Assumes a single, constant business-use percentage for the full period — it does not model a percentage that changes month to month or season to season.
- Does not apply category-specific rules on top of the split — for example, business meals are generally limited to 50% deductible even after isolating the business share, and vehicle expenses follow their own actual-cost or standard-mileage method.
- Does not verify or generate the documentation (mileage log, itemized statements) that supports the business-use percentage — that record-keeping happens outside this tool.
Frequently asked 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 percentage must reflect actual, documented use, not a convenient round number.
What expenses are commonly mixed-use?
Cell phone and internet service, a personal vehicle used for client visits, a laptop or camera, software used for both work and personal projects, and utilities tied to a home office are the usual examples. Each is deducted only to the extent it serves the business.
How do I prove my business-use percentage?
Keep contemporaneous records: a mileage log for a vehicle, a few representative months of itemized phone or internet use, or a usage log for shared equipment. The IRS expects a reasonable, supportable basis for the percentage, and a suspiciously round number without records is a common audit flag.
Is the business portion always fully deductible once I split it?
The split is the first step, not the last. Some categories carry additional limits on top of it — business meals are generally 50% deductible, and vehicles must use either actual costs or the standard mileage rate, not a mix of both. Treat this result as the business share, then apply any category-specific rule.
Does this calculator cover state tax rules too?
No — this estimate reflects the federal business-vs-personal allocation concept described in IRS guidance for Schedule C filers. State income tax treatment of mixed-use business expenses can differ, so confirm your state’s rules separately.