How to Evaluate a Rental Property Before You Buy

Why "Rent Covers the Mortgage" Is an Incomplete Question

It's the first calculation almost every new rental investor runs: take the expected monthly rent, subtract the mortgage payment, and if there's money left over, call it a good deal. It feels like sound logic, and it's exactly the kind of shortcut that leads people into rental properties that quietly lose money for years before anyone notices.

The mortgage payment is only one of the costs a rental property generates. A more complete list of the operating expenses that eat into what looks like "profit" on a rent-minus-mortgage napkin calculation includes:

• Property taxes — often the single largest recurring cost after the mortgage, and one that tends to rise over time.
• Insurance — landlord insurance policies typically cost more than a standard homeowner's policy on a comparable property.
• Maintenance and repairs — a reasonable rule many investors use is budgeting somewhere around 1% of the property's value per year for ongoing upkeep, though this varies with the property's age and condition.
• Vacancy — no rental is occupied 100% of the time. Even a well-managed property will sit empty between tenants occasionally, and that lost rent needs to be budgeted for, not treated as a surprise when it happens.
• Property management — if you're not self-managing, expect a percentage of collected rent (commonly in the 8–10% range, though it varies by market and service level) to go to a management company.

Once all of those are subtracted from rental income, what's left is a much more honest picture of the property's performance than "rent minus mortgage payment" ever was. That honest picture is what the two metrics below are built to capture.

Cap Rate: The Unlevered Metric

The capitalization rate, or cap rate, measures a property's return based purely on its operating performance — completely independent of how it's financed. It answers the question: "if I bought this property with all cash, what yield would it generate?"

The formula is: Cap Rate = Net Operating Income (NOI) ÷ Property Purchase Price. Net operating income is your rental income minus operating expenses (taxes, insurance, maintenance, vacancy, management) — but critically, before subtracting mortgage payments, since cap rate is designed to strip financing out of the picture entirely.

Worked Example

Consider a property purchased for $300,000 that generates $28,000 in annual rental income. Operating expenses — property tax, insurance, maintenance, vacancy allowance, and management — total $10,000 per year.

• Net Operating Income (NOI) = $28,000 − $10,000 = $18,000
• Cap Rate = $18,000 ÷ $300,000 = 6.0%

That 6.0% figure tells you how this property performs on its own merits, regardless of whether you paid cash, took out a mortgage with 20% down, or financed it some other way. Because it strips out financing, cap rate is the metric that lets you compare two entirely different properties — in different markets, at different price points — on an apples-to-apples basis.

Cash-on-Cash Return: The Levered Metric

Cash-on-cash return answers a different, more personal question: "given how I actually financed this deal, what return am I getting on the cash I actually put in?" It reflects your specific financing — down payment, loan terms, and mortgage payment — rather than the property's performance in the abstract.

The formula is: Cash-on-Cash Return = Annual Pre-Tax Cash Flow ÷ Total Cash Invested. Annual cash flow here is NOI minus the annual mortgage payment (principal and interest) — the actual cash left in your pocket each year. Total cash invested is your down payment plus closing costs and any upfront repair or renovation costs.

Same Example, Now With Financing

Let's use the same $300,000 property with $18,000 in NOI, but now assume you put 20% down ($60,000) and financed the remaining $240,000 with a mortgage that carries an annual principal-and-interest payment of $14,400.

• Annual cash flow = NOI − mortgage payment = $18,000 − $14,400 = $3,600
• Total cash invested = $60,000 down payment + $6,000 closing costs (assumed) = $66,000
• Cash-on-Cash Return = $3,600 ÷ $66,000 = 5.5%

Notice this number (5.5%) is different from the cap rate (6.0%) we calculated for the exact same property. That gap is the effect of leverage — the mortgage is doing work in this deal, and cash-on-cash return is what captures its actual impact on your out-of-pocket return. Change the down payment, the interest rate, or the loan term, and the cash-on-cash return changes with it, even though the cap rate — tied to the property's own operating performance — stays exactly the same.

Why Investors Track Both, Not Just One

It's tempting to pick a favorite metric and stop there, but each one is answering a different question, and using only one leaves a blind spot.

• Cap rate is your tool for comparing deals apples-to-apples — screening multiple properties, or comparing a property against typical cap rates in a given market or neighborhood, without financing details muddying the comparison.
• Cash-on-cash return is your tool for evaluating your specific situation — this particular financing arrangement, with this down payment and this loan, and whether it's putting your actual invested cash to good use compared to other places you could put it.

A property with an unremarkable cap rate can still produce an attractive cash-on-cash return if financed well (or vice versa) — which is exactly why looking at only one number can lead you to pass on a good deal or take a bad one. Run both before you commit.

What a Spreadsheet Can't Tell You

It's worth saying plainly: these metrics are built from assumptions, and real rental properties don't always cooperate with the assumptions in a spreadsheet. Vacancy can run longer than budgeted. A roof or HVAC system can fail years ahead of schedule and well beyond a routine maintenance line item. Local rent growth can stall or reverse during a market downturn, and property values themselves move in cycles that don't always go up.

None of that means the numbers aren't worth calculating — quite the opposite. It means treating cap rate and cash-on-cash return as a disciplined starting point for evaluating a deal, not a guarantee of how it will perform. Building in a reasonable margin of safety on your expense and vacancy assumptions is usually wiser than optimizing for the best-case scenario.

Run Your Own Numbers

Our rental property ROI calculator walks through both cap rate and cash-on-cash return for a property you're considering, using your actual purchase price, rent, expenses, and financing terms.

If you're still working out the mortgage side of the equation, our mortgage payment calculator can help you pin down the exact principal-and-interest figure to plug into your cash-on-cash calculation. And if you're weighing this investment decision against simply buying a home to live in, note that our rent vs. buy calculator is built for a different question entirely — it's for a personal home-buying decision, not an investment analysis, so keep the two separate when you're deciding which tool fits what you're actually evaluating.

A rental property that pencils out on both metrics, with reasonable and slightly conservative assumptions, is a far more defensible purchase than one that merely covers its mortgage on a good month.