What Makes a Rental Property a Good Deal? Cap Rate, Cash Flow, and the Numbers That Matter

"The rent covers the mortgage" is the most common way people talk themselves into a bad rental property purchase. It's not that the statement is false — it's that it ignores every other cost of owning the property, which is exactly where the real numbers usually fall apart. Real investors evaluate a deal with a small set of specific metrics, not a gut feeling about whether rent sounds like "enough."

Cash flow: what's actually left after everything

Cash flow is monthly rental income minus every real monthly cost — mortgage payment (principal, interest, tax, insurance), maintenance reserve, vacancy reserve, property management if you use it, and any HOA. Not just the mortgage. A property with $400/month of "profit" against the mortgage alone can easily be negative once maintenance and vacancy are actually budgeted in, rather than hoped away.

A property that's break-even or slightly negative on paper monthly cash flow can still be a reasonable buy if you're underwriting it for appreciation or principal paydown — but that should be a deliberate decision, not something you discover after closing.

Cash-on-cash return: return on the money you actually put in

Cash-on-cash return is annual pre-tax cash flow divided by total cash invested (down payment, closing costs, and any immediate repair costs) — not the property's full purchase price. It answers the question that actually matters to your bank account: what am I earning on the dollars I put in, versus leaving them in an index fund instead.

There's no single "good" cash-on-cash number that applies everywhere — it depends heavily on market, financing terms, and what else you'd do with the capital — but it's the metric that lets you compare two very different properties on equal footing, since it accounts for how much leverage each one uses.

Cap rate: comparing properties independent of financing

Cap rate (net operating income divided by purchase price, ignoring the mortgage entirely) strips out financing so you can compare properties as if you'd bought them in cash. This is the number experienced investors quote first, because it isolates the property's own performance from whatever loan terms happen to be available right now.

A property with a strong cap rate but weak cash-on-cash return usually means the financing terms are the problem, not the property — worth knowing before you write it off, since a different loan structure might make the same property work.

Get the real monthly payment before you run any of this

Every metric above depends on an accurate mortgage payment — including the property tax and insurance most people forget to fold into their first-pass math, on an investment property specifically (rates and insurance costs typically run higher than an owner-occupied home).

Protecting the numbers once you own it

Once cash flow is real money coming in every month, the biggest risk to it is an uninsured or underinsured event — a burst pipe or a liability claim can wipe out years of cash flow in one bad month if the landlord policy isn't right for the property.

If you're evaluating more than one property

Running these numbers by hand across several properties gets tedious fast, and it's easy to lose track of which assumptions you used for which address:

The short version: "the rent covers the mortgage" isn't underwriting. Run the full monthly cost, the cash-on-cash return, and the cap rate before you decide a property is a good deal — the difference between those numbers and a gut feeling is usually the difference between a good investment and a very expensive lesson.