How the Rental Property Calculator works (the math behind the numbers)

Rental real estate looks simple (“rent minus mortgage”) until you add the stuff that actually determines whether a deal is good: vacancy, repairs, capex, taxes, insurance, management, and financing. This calculator estimates your monthly cash flow, annual cash flow, cap rate, cash-on-cash return, DSCR, and a simple 5-year total return projection so you can compare deals quickly.

If you finance a property, your loan amount is: Loan Amount = Purchase Price − Down Payment. The monthly principal-and-interest payment is estimated using the standard amortization formula: P = L × r × (1 + r)ⁿ / ((1 + r)ⁿ − 1) where L is the loan amount, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of monthly payments (loan term in years × 12).

Real rentals aren’t occupied 100% of the time. We model vacancy as a percentage of gross rent: Vacancy Loss = Gross Monthly Rent × Vacancy %. Your Effective Gross Income (EGI) becomes: EGI = (Rent + Other Income) − Vacancy Loss. Other income can include parking, laundry, storage fees, or pet rent.

Operating expenses are everything required to operate the property (not including mortgage principal/interest). We include a mix of fixed monthly items (taxes, insurance, HOA, utilities you pay) and percentage-based reserves: Maintenance (% of rent), CapEx reserve (% of rent), and Management (% of EGI). These percentages are useful because expenses tend to scale with rent and property size.

NOI is the core real-estate profitability metric. It ignores financing and looks only at property operations: NOI = (EGI × 12) − (Operating Expenses × 12). NOI is what investors use for cap rate and many valuations.

Cap rate helps compare properties independent of your financing: Cap Rate = NOI / Purchase Price. Cap rate is great for comparing similar assets in a market, but it doesn’t capture your leverage (down payment) or your cash flow after debt.

Monthly Cash Flow is the number that decides whether the property pays you or you pay it: Monthly Cash Flow = EGI − Operating Expenses − Monthly P&I. If this is negative, your deal may still “work” as a long-term appreciation play, but your risk is higher because you must fund the shortfall.

Cash-on-cash return measures return on the cash you actually put into the deal (down payment + closing + rehab, etc.). We estimate: Cash-on-Cash = Annual Cash Flow / Total Cash Invested. Many investors consider 6–12% cash-on-cash healthy, but the “right” number depends on market stability, appreciation expectations, and your personal risk tolerance.

DSCR is a lender-friendly metric: DSCR = NOI / Annual Debt Service. Annual debt service here is your monthly P&I × 12. A DSCR above 1.0 means the property’s NOI covers the mortgage payment. Many DSCR loans prefer around 1.15–1.25+ depending on the lender and product.

To make this calculator more “decision-friendly,” we add a simple 5-year projection using your slider assumptions: appreciation, rent growth, and expense growth. We estimate property value after 5 years, total net cash flow over 5 years, and an approximate “equity build” from mortgage amortization. This is not a guarantee—think of it as a sandbox for testing conservative vs optimistic scenarios.

Tip: Use three scenarios: Conservative (higher vacancy + higher expenses + lower appreciation), Base, and Aggressive. Save them to compare like a mini investment memo.

A realistic “sanity-check” example

Suppose you’re analyzing a $300,000 rental. You put 20% down ($60,000), pay $9,000 closing costs, and budget $6,000 in minor repairs/updates. That’s $75,000 cash invested.

You expect $2,400/mo rent and $0 other income. You assume 6% vacancy, 8% maintenance reserve, 8% capex reserve, and 8% management (if self-managing, you can set this to 0–5%). Property taxes are $350/mo, insurance $125/mo, and HOA $0. You finance the remaining $240,000 at 6.75% for 30 years.

The calculator will estimate your monthly mortgage payment, subtract vacancy, then subtract expenses to find NOI and cash flow. If you get around $150–$300/mo in cash flow, your cash-on-cash might land in the 2–5% range. That’s not “bad” in expensive markets—many investors accept low cash flow if they expect long-term appreciation and rent growth. If you get $600–$900/mo in cash flow, you’re likely looking at a stronger cash-flow market (or a true value-add deal).

Use the Deal Score to quickly compare properties. A high score typically means positive cash flow, solid DSCR, and healthy cash-on-cash return. A low score means either the property is thin (barely pays for itself) or your assumptions are conservative. Change one thing at a time (rent, purchase price, down payment, vacancy) and watch which lever moves the deal most.

How to evaluate a rental deal step-by-step

• Start with reality: enter purchase price, expected rent, and real taxes/insurance from listings or county data.
• Pick conservative sliders: vacancy 6–10%, maintenance 6–10%, capex 5–10%. Conservative inputs prevent “deal hallucinations.”
• Check cash flow first: if you’re negative, ask whether you can comfortably fund that gap for years.
• Then check DSCR: if DSCR is under 1.0, lenders may dislike it and you’re relying on appreciation to win.
• Use cap rate for market comparison: compare to similar properties in that neighborhood/city.
• Use cash-on-cash for your personal plan: if you want income, prioritize stronger CoC return.
• Save scenarios: create Conservative / Base / Aggressive snapshots and compare them side-by-side.

If you’re building a portfolio, consistency matters: use the same assumptions across deals so your comparisons are apples-to-apples.

Frequently Asked Questions

• What’s the difference between NOI and cash flow?

  NOI excludes financing (mortgage). Cash flow includes mortgage payments. NOI tells you how the property performs as a business; cash flow tells you how it impacts your monthly budget.

• Should I include mortgage principal as an “expense”?

  For cash flow, yes (principal reduces cash). For NOI, no (NOI is before debt service). Principal is not “lost”—it builds equity.

• What vacancy rate should I assume?

  Many investors use 5–10% as a default. Higher-turnover properties or weaker neighborhoods may need more. If you’re unsure, use 8–10%.

• What is CapEx?

  CapEx is capital expenditures—big, infrequent items like roofs, HVAC, water heaters, exterior paint, or major plumbing. A CapEx reserve helps prevent surprise “portfolio-killer” repairs.

• Does this include income taxes, depreciation, or tax benefits?

  No. Those depend on your personal situation. This calculator focuses on deal fundamentals. Use a tax pro for depreciation and deductions.

• How accurate is the 5-year projection?

  It’s a planning tool, not a prediction. Markets can outperform or underperform. Use it to compare scenarios and stress-test assumptions.