Before you make an offer on a rental property, run five numbers: net operating income, cap rate, cash-on-cash return, debt service coverage ratio, and the expense ratio. Together they tell you whether a property is a business that pays you or a liability that happens to have tenants in it. None of them require more than a spreadsheet and honest inputs — the skill is refusing to use the seller's optimistic numbers instead of your own.
What changed in 2026
- Insurance became the line item that breaks deals. In many coastal and wildfire-exposed markets, premiums now run several times what they did five years ago, and that alone can flip a property from cash-flow positive to negative.
- Rent growth cooled in most metros after the sharp increases of 2021-2023, so underwriting aggressive annual rent growth is no longer a safe default assumption.
- Portfolio and DSCR lenders became a bigger share of investor financing, which means loan-level math matters even for buyers who would once have used a conventional mortgage.
- Rent and comp data got easier to verify through public rent-estimate tools, reducing the excuse for underwriting off a seller's pro forma sheet.
The five numbers to run
Net operating income (NOI). Gross rental income minus operating expenses (property tax, insurance, maintenance, property management, vacancy allowance) — before the mortgage payment. This is the number every other metric is built on.
Cap rate. NOI divided by purchase price. A quick, financing-independent measure of return. Useful for comparing deals; weak on its own because it ignores your specific loan terms. See a full breakdown in cap rate explained for 2026.
Cash-on-cash return. Annual pre-tax cash flow (NOI minus mortgage payments) divided by the actual cash you invested (down payment plus closing costs plus initial repairs). This is the number that reflects your leverage, not just the asset.
Debt service coverage ratio (DSCR). NOI divided by annual mortgage payment. Most lenders want 1.20x or higher — meaning the property produces at least 20% more income than the loan payment requires. Below 1.0x, the property cannot cover its own debt.
Expense ratio. Operating expenses divided by gross rental income. A common rule of thumb is that expenses (excluding the mortgage) run 40-50% of gross rent once you include a realistic vacancy and maintenance reserve, even on newer properties.
Worked example: a $300,000 rental
Hypothetical numbers, rounded for clarity:
| Line item |
Monthly |
Annual |
| Gross rent |
$2,200 |
$26,400 |
| Vacancy reserve (5%) |
-$110 |
-$1,320 |
| Property tax |
-$250 |
-$3,000 |
| Insurance |
-$150 |
-$1,800 |
| Maintenance reserve |
-$180 |
-$2,160 |
| Property management (8%) |
-$176 |
-$2,112 |
| NOI |
$1,334 |
$16,008 |
| Mortgage payment (P&I) |
-$1,450 |
-$17,400 |
| Cash flow |
-$116 |
-$1,392 |
Cap rate: $16,008 / $300,000 = 5.3%. DSCR: $16,008 / $17,400 = 0.92x — below the 1.20x most lenders require, and cash flow is negative. Cash-on-cash on $80,000 total cash in (down payment plus closing and initial repairs): -$1,392 / $80,000 = -1.7%.
This property fails on DSCR and cash-on-cash despite a respectable-looking cap rate — exactly the kind of deal that looks fine on a one-line summary and falls apart on the full worksheet.
Common mistakes
Underwriting the seller's pro forma rent. If the listing rent assumes renovations, a new lease-up, or short-term rental income you have not verified, replace it with the trailing 12-month actual rent or a conservative comp-based estimate.
Forgetting vacancy and capex reserves. A property that "cash flows" with zero vacancy and no reserve for the roof or HVAC is not cash-flowing — it is deferring the expense to a future year.
Using cap rate alone to compare deals across markets. Cap rates differ by market for reasons that include real risk (property tax rates, appreciation expectations, tenant quality). A higher cap rate is not automatically a better deal.
Ignoring DSCR until the lender brings it up. Run DSCR yourself before you get attached to a property. A deal that cannot hit 1.20x may not close even if you love it.
FAQ
What is a good cap rate for a rental property?
It depends heavily on market and property type — a number that looks low in a stable, low-risk market can be a good deal, while the same number in a declining market is a warning sign. Compare within a market, not across the country.
What NOI margin should I expect?
Many rentals run an expense ratio of 40-50% of gross rent once vacancy, maintenance, and management are included, leaving an NOI margin of roughly 50-60%. New or self-managed properties can do better; older properties often run worse.
Is cash-on-cash return more important than cap rate?
For a leveraged buyer, yes — cash-on-cash reflects your actual loan terms and the cash you put at risk. Cap rate is useful for a quick, apples-to-apples first screen before you model financing.
Do I need a DSCR above 1.20x if I am not using a DSCR loan?
Even with a conventional mortgage, a DSCR below 1.0x means the property does not cover its own debt from operations, which is a warning sign regardless of how the loan is underwritten.
Where to go next
For related reading, see Real estate investing for beginners in 2026, Cap rate explained for 2026, and 1031 exchange explained for 2026 for what happens when you eventually sell.