Cash flow is the oxygen of a rental portfolio. It's what keeps you in the game when vacancies hit, repairs surprise you, and markets soften. But most beginner investors calculate cash flow incorrectly — they subtract only the mortgage payment from the rent and call the difference "profit." Real cash flow calculation is more thorough, and the difference between the two numbers determines whether your deal is genuinely profitable or a slow drain on your bank account.

The Full Cash Flow Calculation

Here's the complete formula, step by step:

  1. Gross Rental Income: Monthly rent × 12 months = annual gross rents
  2. − Vacancy Allowance (5-8%): Budget for months between tenants, evictions, and downtime
  3. = Effective Gross Income (EGI)
  4. − Operating Expenses: Property taxes + insurance + PM fee (8-10%) + maintenance (8-10%) + CapEx reserves (5-8%) + utilities (if landlord-paid) + HOA (if applicable)
  5. = Net Operating Income (NOI)
  6. − Debt Service (PITI): Principal + Interest + Taxes (if escrowed) + Insurance (if escrowed)
  7. = Monthly Cash Flow

A Real Cash Flow Example

Property: $220,000 purchase price, $44,000 down (20%), $176,000 loan at 7.25%, 30 years. Monthly rent: $1,800.

This deal looks good on the surface ($1,800 rent − $1,198 mortgage = $602 "profit") but is actually cash flow negative when all expenses are included. This is why proper calculation matters.

CapEx Reserves Are Not Optional: Capital expenditure reserves (for roof replacement, HVAC, water heater, appliances) are real costs that just don't come due every month. Budget 5-10% of gross rents annually. A landlord who skips CapEx reserves will face a $12,000 roof bill with no savings — and either go into debt or defer maintenance until it becomes a major problem.

Get Your True Cash Flow Number in 30 Seconds

The Property Analyzer handles the full cash flow model automatically — vacancy, maintenance, CapEx, management, and all financing costs.

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What Separates Good Cash Flow Markets from Bad Ones

Cash flow is primarily driven by the rent-to-price ratio. The higher the rent relative to purchase price, the easier it is to generate positive cash flow after all expenses. In 2026, with 7%+ mortgage rates, most coastal markets (Los Angeles, New York, Seattle) are impossible to cash flow without very large down payments — the math doesn't work at 20% down. Midwest and Southeast markets (Columbus, Indianapolis, Birmingham, Memphis) still offer rent-to-price ratios that support positive cash flow.

Use the 1% Rule as a quick screening tool: if monthly rent is at least 1% of purchase price, the deal has a chance of cash flowing with conventional financing. Below 0.7%, it almost certainly won't at current rates.

Cash Flow vs. Total Return — What Really Matters

Cash flow is important, but it's not the only return in real estate. The four return components are: cash flow, appreciation, equity paydown (your tenant pays down your mortgage), and tax benefits (depreciation). A deal with modest cash flow in a strong appreciation market may outperform a high-cash-flow deal in a stagnant market over 10 years. See cash-on-cash return for the metric that captures your actual return on invested capital.