Updated June 17, 2026
To analyze a rental property, estimate gross rental income, subtract a vacancy allowance, subtract all operating expenses to find NOI, calculate cap rate (NOI divided by purchase price), subtract debt service to find cash flow, then calculate cash-on-cash return (annual cash flow divided by total cash invested). Run a stress test before making an offer.
Why Underwriting Matters Before You Make an Offer
Many first-time investors skip formal analysis and rely on the seller's pro forma, which almost always shows the property in the best possible light. Sellers may omit vacancy allowances, use below-market expense estimates, or count one-time income as recurring. A disciplined underwriting process forces you to build your own numbers from the ground up.
For this walkthrough, we will use a hypothetical duplex listed at $320,000 in a mid-size U.S. city, where each unit rents for $1,100 per month.
Step 1: Estimate Gross Rental Income
Start with what the property could earn at full occupancy. Use actual current rents if the property is occupied, or comparable rents from similar units nearby if it is vacant. Do not accept the seller's rent estimate without verifying it against real listings and recent leases.
Example: Two units at $1,100/month each = $2,200/month = $26,400/year gross potential income.
If you have access to market rent data from local property managers or rental listing sites, cross-check the asking rents against that data. Overestimating rent is one of the most common mistakes in rental property analysis.
Step 2: Apply a Vacancy and Credit Loss Allowance
No property stays fully occupied every month, every year. Leases turn over, tenants miss payments, and units sit vacant during transitions. A standard allowance is 5% to 10% of gross potential income, depending on local vacancy rates and the property's tenant history.
Example: 7% vacancy allowance on $26,400 = $1,848. Effective gross income = $26,400 - $1,848 = $24,552/year.
Step 3: List and Total All Operating Expenses
This is where many investors underestimate costs. Operating expenses include all costs to run the property, excluding mortgage payments and income taxes. A reliable expense checklist includes:
- Property taxes (verify with the county assessor; taxes often reassess after a sale)
- Insurance (landlord/dwelling policy, not homeowner)
- Property management (typically 8% to 12% of collected rent if you use a manager)
- Maintenance and repairs (budget 1% of property value per year as a starting point)
- Capital expenditure reserve (roof, HVAC, appliances; budget separately from routine maintenance)
- Utilities paid by landlord (water, trash, common-area electric)
- Landscaping, pest control, or other services
These expenses, when paid or accrued, are generally deductible and reported on IRS Schedule E at tax time. The IRS also details which rental costs qualify as deductions at Tax Topic 414 (Rental Income and Expenses).
Example expenses for our duplex:
| Expense | Annual Amount |
|---|---|
| Property taxes | $3,800 |
| Insurance | $1,200 |
| Maintenance (1% of value) | $3,200 |
| CapEx reserve | $1,500 |
| Property management (9%) | $2,210 |
| Misc. (landscaping, trash) | $600 |
| Total Operating Expenses | $12,510 |
Step 4: Calculate Net Operating Income and Cap Rate
Subtract total operating expenses from effective gross income to get NOI. Then divide NOI by the purchase price to get the cap rate.
NOI = $24,552 - $12,510 = $12,042
Cap Rate = $12,042 / $320,000 = 3.76%
This cap rate is on the lower end for a mid-size market. Before walking away, check what comparable duplexes in the same neighborhood are trading at. If the market cap rate for similar properties is 4.5%, this property is overpriced for its income and you have a basis for negotiation. Use our cap rate calculator to test different price points and see how the cap rate responds.
Step 5: Calculate Cash Flow and Cash-on-Cash Return
Now layer in your financing. Assume a 25% down payment ($80,000) plus $6,000 in closing costs, for a total cash investment of $86,000. Your mortgage on $240,000 at a 7% rate over 30 years is approximately $1,597/month, or $19,164/year.
Annual Cash Flow = NOI - Debt Service = $12,042 - $19,164 = -$7,122
Cash-on-Cash Return = -$7,122 / $86,000 = -8.3%
This property has negative cash flow at current price and financing terms. That does not automatically make it a bad investment if you expect strong appreciation, but you need to fund the shortfall out of pocket every month. Model how many years of appreciation it would take to break even on a total-return basis.
Step 6: Stress-Test the Numbers
Before you commit, run at least two downside scenarios:
- Higher vacancy: What happens if vacancy is 15% instead of 7%? Recalculate NOI and cash flow.
- Rent decline: What if rents fall 10% at next lease renewal? Many markets have seen rent softening after periods of rapid growth.
- Major repair: What if the roof needs replacement in year two for $12,000? Does your reserve cover it, or does it wipe out multiple years of cash flow?
- Rate increase at refinance: If you plan to refinance in five years, what does cash flow look like at a higher rate?
If the deal still makes sense under two or three downside scenarios, you have identified a margin of safety. If it only works under the optimistic base case, consider whether the risk is appropriate for your financial situation.
Pulling It All Together
Rental property analysis is not complicated, but it requires discipline and honest assumptions. The investors who lose money on rentals almost always did so because they used the seller's numbers, skipped the expense categories they hoped would not apply, or modeled only the best case. Build your own spreadsheet or use a dedicated calculator, verify every income and expense input against third-party sources, and run your stress test before you submit an offer. That process, repeated consistently, is how long-term investors build portfolios that actually cash flow.