Monthly rent income

Other monthly income

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Property tax + insurance

Maintenance + vacancies

Management + other

Reserve for vacancy, repairs, management, and capital expenditures. Taxes and insurance vary by market.

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Duplex cash flow example showing income, expenses and thin monthly surplus — The duplex clears just +$95 per month before tax.

Method & verification

How This Tool Works

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Calculation Method

Computes Monthly cash flow from Monthly rent, Other monthly income, Monthly mortgage, Monthly maintenance and Other monthly expenses.

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Worked example: a $1,400 rent duplex unit

Suppose rent is $1,400 and a shared laundry machine brings in $50, so gross income is $1,450. On the cost side: mortgage principal and interest $890, tax and insurance combined $260, maintenance set aside $120, and misc (HOA, utilities you pay) $85. Total expenses are 890 + 260 + 120 + 85 = $1,355.

Cash flow is 1,450 - 1,355 = +$95 per month, or $1,140 per year. That looks thin for the risk, and it is: the $120 maintenance line is only about 8% of rent. A single $1,800 furnace repair wipes out more than a year of that surplus. Bumping maintenance to the common 10% rule of thumb ($140) drops the result to +$75 per month, and one vacant month at $1,400 rent turns the year into roughly -$260 unless the vacancy fund covers it.

Where this simple model breaks down

The calculation is pre-tax and pre-vacancy by design. A unit with $95 of monthly surplus and a 25% marginal rate does not owe tax on all of it: mortgage interest and depreciation (roughly 3.6% of a $200,000 structure per year, about $600 per month of paper loss) usually shield it, but that same depreciation is recaptured at sale. The tool shows the operating number, not the after-tax one.

Timing matters too. Expenses here are monthly averages, but real costs arrive in lumps: a $7,000 roof on a duplex is 73 months of that $95 surplus. CapEx reserves of $200 to $300 per unit per month are standard for older buildings and are missing from the maintenance field unless you enter them there. Also note income is assumed constant; a 5% rent bump to $1,470 raises monthly flow to $115, a 21% gain, while a 5% expense rise cuts it to $49, nearly half. Small properties amplify percentage changes on thin margins.