How to Analyze Rental Property Cash Flow (Step-by-Step)
Why cash flow is the number that matters
Cash flow is the money left in your pocket each month after every expense is paid. Appreciation is a bonus you can't control; cash flow is what keeps a rental alive. A property that loses $200 every month is a liability with a mortgage attached — no matter how much it might appreciate.
Before buying any rental, you should be able to answer one question with confidence: after rent comes in and every bill goes out, how much is left?
Step 1 — Start with realistic rental income
Your starting point is the monthly rent the property will actually command — not the seller's optimistic number. Check comparable rentals in the same neighborhood with similar bedrooms and condition. If the property qualifies for Section 8 tenants, look up the HUD Fair Market Rent for the ZIP code: it's a reliable baseline that housing authorities actually pay.
Add other income if it exists (parking, laundry, storage), but never count on it to make a deal work.
Step 2 — Subtract operating expenses (the part most people underestimate)
Operating expenses are everything it costs to run the property, excluding the loan: property taxes, insurance, property management (8-10% of rent), maintenance and repairs (budget 5-10% of rent even on renovated homes), and vacancy (5-8% of rent set aside for the months nobody pays).
A common shortcut is the 50% rule: over the long run, operating expenses tend to eat roughly half of the rent on older properties. It's not a substitute for real numbers, but if your estimate of expenses is far below 40-50% of rent, you're probably missing something.
Step 3 — Subtract debt service
Debt service is your monthly mortgage payment: principal and interest. What's left after operating expenses AND debt service is your net cash flow. The industry also measures this as DSCR (Debt Service Coverage Ratio): net operating income divided by debt service. Lenders typically want a DSCR of at least 1.2 — meaning the property earns 20% more than the loan payment.
Step 4 — Judge the result like an investor
A positive number isn't automatically a good deal. Compare the annual cash flow against the cash you invested (down payment, closing costs, initial repairs) — that's your cash-on-cash return. Many buy-and-hold investors target $150-$300+ per month per door and a cash-on-cash return above 8-10%, but your market and strategy set the bar.
Run the numbers with honest inputs, then stress-test them: what happens if rent drops 10%, or a $3,000 repair hits in year one? A deal that only works in the best-case scenario doesn't work.
FAQ
What is a good cash flow for a rental property?
Many investors target at least $150-$300 per month per unit after all expenses and the mortgage, though targets vary by market and price point. The percentage view matters too: annual cash flow divided by cash invested (cash-on-cash return) above 8-10% is a common benchmark.
What expenses do new investors forget most often?
Vacancy, maintenance reserves, and property management. Even if you self-manage today, budgeting 8-10% for management keeps the deal honest — and keeps your option to step away open.
What is the 50% rule?
A rough screening shortcut: assume operating expenses (excluding the mortgage) will consume about 50% of rent over the long run. Use it to filter deals quickly, then replace it with real numbers before buying.