How Do You Calculate the Business-Use Percentage for a Shared Expense?
A car, a phone, a home office — plenty of costs that support a business also do double duty in everyday personal life, which is exactly why a business-use percentage exists in the first place.
The short answer
A business-use percentage is an estimate of how much of a shared cost is attributable to business activity versus personal use, applied to the total expense to figure out the deductible portion. There isn’t one single formula that works for every kind of expense — the right method depends on what’s being measured, whether that’s time, space, or mileage. What matters most across all of them is that the method is reasonable and applied consistently.
Time-based allocation
For costs tied to how something is used over time — a phone plan or an internet subscription, for example — the allocation is often based on the share of actual use that’s business-related. This might come from reviewing call or usage logs over a representative period, or making a reasonable estimate based on a typical week or month of work. It’s the approach that tends to apply to something like home internet and phone costs, where the bill itself doesn’t separate business calls from personal ones.
Space-based allocation
For costs tied to physical space — most commonly a portion of a home used for business — the allocation is often based on square footage, comparing the area used regularly and exclusively for business against the total space of the home. This is the logic behind a home office deduction, where the percentage of the home dedicated to business use becomes the basis for allocating a share of costs like rent, utilities, or a mortgage.
Mileage-based allocation
For a vehicle used for both business and personal driving, the allocation is typically based on miles driven, comparing business miles against total miles for the year. This is closely tied to how a standard mileage rate deduction works, since that approach relies on an accurate log of business versus personal driving rather than allocating a flat percentage of total vehicle costs.
Why consistent recordkeeping matters across all of them
Every one of these methods depends on some kind of ongoing record — usage logs, square footage measurements, or a mileage log — rather than a one-time estimate made after the fact. A percentage that’s reconstructed from memory months later is much harder to support than one built from records kept along the way. Consistency matters too: a business-use percentage that changes dramatically from year to year without a clear reason, like a change in how a space or vehicle is actually used, can be harder to defend than one that stays relatively stable. These allocated costs typically get reported together with other business expenses on a Schedule C.
A practical habit
Whether the expense in question is a phone bill, a home office, or a vehicle, the underlying question is the same: what share of this cost is genuinely tied to business activity, based on a reasonable and defensible measure? Building a simple habit of logging usage, space, or mileage as it happens — rather than estimating after the fact — tends to produce a number that holds up better than a guess. Because these rules and acceptable methods can vary by situation, it’s worth treating this as a general framework to apply thoughtfully rather than a single fixed formula.