Make the result useful
Shared-expense allocation
Shared expense is the total amount that must be allocated.
Rental-use percentage is the method you choose and must be supportable.
Rental portion is the calculated share; personal portion is the remainder.
Keep the allocation method and source records with the transaction.
The assumptions that move this result
Shared expense
Full mixed-use cost.
Rental percentage
Supportable rental-use share.
Rental portion
Calculated allocation.
Personal portion
Remaining non-rental share.
rental portion = shared expense × rental-use percentage
Use the output as a documented scenario result, not a guarantee.
Read the number in context
Worked scenario
Scenario: $1,200 × 60% = $720 rental portion.
Edge case
Edge case: usage can change during the year, requiring a revised basis.
Not tax advice and does not establish an allowable deduction.
Before you act
• Document the allocation basis.
• Keep invoices and payment records.
• Confirm treatment with a tax professional.
Worked formula
rental portion = shared expense × rental-use percentage
Is this a forecast?
No. It calculates the assumptions you enter.
Can it replace professional review?
No. Use current records and qualified advice.
What should I save?
Keep the assumptions and source records used for the decision.