How to Calculate Monthly Cash Flow on a Rental Property (The Right Way)

The Formula Most Investors Get Wrong

The back-of-napkin version of rental cash flow is rent minus mortgage. It is also wrong.

It ignores six expense categories that every income property carries. When those categories go unmodeled, a property that looks like it generates $400 per month in cash flow often generates $40. Sometimes it generates negative cash flow.

The correct formula has three stages.

Stage 1: Gross Potential Rent (GPR) Start with the monthly rent at 100% occupancy.

Stage 2: Effective Gross Income (EGI) Subtract vacancy and credit loss. The national average vacancy rate for single-family rentals sits at 6.4% (U.S. Census Bureau, 2024). Apply that rate to GPR.

Stage 3: Net Operating Income (NOI) Subtract all operating expenses from EGI. Operating expenses do not include mortgage payments.

Stage 4: Monthly Cash Flow Subtract your monthly debt service (principal plus interest) from NOI.

That is the complete chain. Every shortcut between Stage 1 and Stage 4 is a guess.

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What Counts as an Operating Expense

This is where most landlord spreadsheets collapse. Operating expenses include every cost required to keep the property operational and legally compliant, excluding financing.

The standard categories:

• Property taxes. Divide the annual tax bill by 12.
• Insurance. Landlord policies run 15% to 25% more than standard homeowner policies. Budget $1,200 to $2,400 annually on a single-family home, depending on location and coverage.
• Property management. If self-managing, assign a value anyway. Your time has a cost. Professional management fees run 8% to 12% of collected rent.
• Repairs and maintenance. Budget 1% of property value per year as a floor. A $320,000 property requires $3,200 annually, or $267 per month.
• Capital expenditure (CapEx) reserves. Roof, HVAC, water heater, appliances, flooring. These are not "if" costs. They are "when" costs. Budget an additional 5% to 10% of gross rent for CapEx reserves.
• Utilities. Any utility the landlord covers, including water, trash, or common-area electricity.
• Landscaping and snow removal. Frequently overlooked in pro formas for single-family rentals.
• Accounting and legal. An LLC, annual filings, and a CPA cost real money. Spread those costs across your units.

Add those categories before you write a single number in the profit column.

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Worked Example 1: Single-Family Rental in the Midwest

Property: 3-bedroom, 2-bathroom single-family home Purchase price: $285,000 Down payment: $71,250 (25%) Loan amount: $213,750 Interest rate: 7.25% (30-year fixed) Monthly principal and interest: $1,458 Gross monthly rent: $1,950

Step 1: Effective Gross Income

Apply a 6.4% vacancy rate.

$1,950 x 0.064 = $124.80 vacancy allowance per month EGI = $1,950 - $124.80 = $1,825.20

Step 2: Monthly Operating Expenses

Step 3: Net Operating Income

$1,825.20 - $1,101.52 = $723.68 NOI

Step 4: Monthly Cash Flow

$723.68 - $1,458 (debt service) = -$734.32

This property loses $734 per month. The gross yield looked attractive. The actual cash flow is deeply negative. An investor using the rent-minus-mortgage shortcut would have calculated $492 per month in positive cash flow and made a catastrophic acquisition decision.

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Worked Example 2: Small Multifamily (Duplex) in the Southeast

Property: Duplex, both units renting at $1,375/month Purchase price: $420,000 Down payment: $105,000 (25%) Loan amount: $315,000 Interest rate: 7.10% (30-year fixed) Monthly principal and interest: $2,119 Gross monthly rent: $2,750 combined

Step 1: Effective Gross Income

$2,750 x 0.064 = $176 vacancy allowance EGI = $2,750 - $176 = $2,574

Step 2: Monthly Operating Expenses

Step 3: Net Operating Income

$2,574 - $1,614.90 = $959.10 NOI

Step 4: Monthly Cash Flow

$959.10 - $2,119 = -$1,159.90

Still negative. However, notice the difference between the two properties. The duplex generates 32.6% more NOI than the single-family home despite only a 48% increase in purchase price. At a lower interest rate or with 30% down, the duplex crosses into positive territory. The single-family home does not.

This is why running the actual math matters. The structure of the deal matters more than the gross rent number.

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When Positive Cash Flow Actually Appears

At current interest rates (7.0% to 7.5% range), pure cash flow positive acquisitions on leveraged residential rentals are rare without one of these conditions:

1. Purchase below market value. Foreclosures, estate sales, and off-market deals can reduce the loan amount enough to shift the math.
2. Large down payment. Reducing leverage reduces debt service. A 35% down payment on the Midwest example above reduces the monthly payment to $1,135 and turns the position cash flow positive by $29. Thin, but positive.
3. High-rent markets with low property tax rates. Texas and California's property tax structures affect this comparison dramatically.
4. Value-add acquisitions. Buying under-rented properties and raising rents to market after improvements is the most common path to cash flow positive returns at current rates.

None of these conditions appear automatically. They require underwriting each deal against the full formula before committing capital.

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The Role of the Expense Ratio in Your Underwriting

Experienced property investors use the expense ratio as a quick filter before running full numbers.

Expense ratio = Total operating expenses / Gross potential rent

A well-maintained, professionally managed single-family rental typically carries an expense ratio between 35% and 50%. A higher expense ratio signals either a high-maintenance property, below-market rents, or a high property tax jurisdiction.

In the Midwest example above: $1,101.52 / $1,950 = 56.5% expense ratio

That ratio is above normal for single-family. It signals the property is not generating enough rent relative to its cost structure. The expense ratio alone would have prompted a closer look before proceeding.

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Three Numbers to Check Before Any Acquisition

1. The 1% Rule (as a filter, not a conclusion) Monthly rent should equal at least 1% of the purchase price. $285,000 purchase price requires $2,850 monthly rent to pass. The Midwest example at $1,950 fails this filter immediately. At current rates, even 1% properties rarely cash flow. Properties below 1% almost never do.

2. Gross Rent Multiplier (GRM) Purchase price divided by annual gross rent. A GRM above 14 typically signals negative cash flow at 75% LTV and current rates. The Midwest property: $285,000 / $23,400 = 12.2 GRM. The duplex: $420,000 / $33,000 = 12.7 GRM.

3. Cash-on-Cash Return Annual cash flow divided by total cash invested (down payment plus closing costs plus any immediate repairs). This metric tells you what your equity is actually earning. A 6% cash-on-cash return is the general threshold for an investment-grade residential rental at current rates.

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Run Your Own Numbers

The formula is fixed. The inputs change for every property, every rate environment, and every local market.

Both worked examples above used assumptions that may not match your target market. Property tax rates, insurance costs, and local vacancy rates vary significantly by ZIP code.

The CalcMoney Mortgage Calculator lets you input your actual loan amount, rate, and term to get precise debt service figures. Pair that output with your expense schedule, and you have a complete cash flow model in under ten minutes.

Build your rental cash flow model in the CalcMoney Mortgage Calculator →

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• How to Calculate ROI on a Rental Property Before You Buy

Every acquisition decision should start there. Not with the gross rent. Not with the neighborhood. With the monthly cash flow number, built from the full formula.