How to Analyze Rental Property Cash Flow Before You Buy

New investors consistently make the same mistake: they estimate rental income, subtract the mortgage payment, and call it cash flow. That number is almost always wrong — and being wrong on cash flow projections is how investors end up losing money on properties they thought were profitable.

The Real Cash Flow Formula

True cash flow = Gross Rental Income − Vacancy Allowance − Operating Expenses − Debt Service.

Every one of those line items matters. Let's walk through each one precisely.

Gross Rental Income

Start with what the market will actually bear, not what you hope to charge. Pull comparable rentals within a half-mile radius, similar bed/bath count, similar condition. Be honest. Optimistic rent projections are a leading cause of underperforming rentals.

Vacancy Allowance

Even the best-run rentals have turnover. Budget 5–8% of gross rents for vacancy — that's roughly 3–4 weeks per year of lost rent. In softer rental markets, push that to 10%. Skipping this line item is wishful thinking.

Operating Expenses

This is where most investors underestimate. A realistic operating expense ratio for a single-family or small multifamily is 35–50% of gross rents. That includes:

Property taxes — get the actual current bill, not an estimate. Insurance — landlord policies run $1,200–$2,500/year depending on property type and location. Property management — if you self-manage now, you may not forever. Budget 8–10% of gross rents. Maintenance and repairs — budget 1% of property value per year for ongoing maintenance. CapEx reserves — budget another 5–10% of gross rents for capital expenditure replacements (roof, HVAC, water heater, appliances). Utilities — for multifamily or properties where you pay water/sewer/trash.

Debt Service

Your monthly principal and interest payment on the mortgage. If you're using a DSCR loan, your lender will qualify the property on this ratio directly — typically requiring 1.1x or 1.25x coverage.

What a Good Cash Flow Number Looks Like

After all expenses and debt service, most buy-and-hold investors target $200–$400/month net cash flow per unit at minimum. More important than the dollar amount is the cash-on-cash return — your annual cash flow divided by your total cash invested. Target 8–12% CoC in most markets.

Run the Numbers Before You Fall in Love

The discipline of running real numbers before emotionally committing to a property separates profitable investors from break-even ones. Use a consistent analysis framework every single time — whether it's a spreadsheet, a calculator tool, or a dedicated platform — and never skip the expense categories that are easy to forget.

Cash flow is not what a property earns. It's what it earns after everything else. Know the difference, and you'll make far better buying decisions.