A property can look like a great deal on paper and still lose you money every single month. The most common mistake is looking at rent minus the mortgage payment and calling that “profit.” That’s not cash flow. That’s just two numbers, ignoring dozens of other costs that show up the moment you own the place.

We own multiple rental properties, and real estate has been one piece of how we built a net worth of $1 million. None of that happened by getting lucky on a purchase. It happened by running the numbers carefully, every time, before making an offer. Below is the simple framework we use to tell whether a rental property is actually a good investment or just looks like one.

1. Start With the Property’s Realistic Rental Income

Don’t Rely Only on the Seller’s Rent Estimate

Sellers and agents sometimes advertise a rent number that isn’t realistic. Before trusting it, research comparable rentals nearby, ones with a similar size, number of bedrooms and bathrooms, condition, and location. If a current tenant is paying less than market rent, don’t assume you can raise it to market rate the moment you buy the property. Turnover takes time, and existing leases matter.

A good place to start that research is online. Zillow lets you search current rental listings in the same neighborhood, filtered by bedrooms, bathrooms, and square footage, so you can see what similar homes are actually renting for right now, not what an owner hopes to get. Zillow’s Rent Zestimate can offer a quick starting point too, though it’s an algorithm-based estimate, not an appraisal, so treat it as a rough guide rather than the final word. Other sites like Rentometer, Realtor.com, and Apartments.com are worth checking as well, since each pulls from slightly different listing data, and cross-referencing a few sources gives you a more reliable range than relying on just one. Pay attention to how long similar listings have been sitting on the market too. A comparable unit that’s been listed for months at a certain price is a sign that price may be too high.

Estimate Vacancy

Don’t assume 100% occupancy. Even great rental properties sit empty between tenants sometimes, whether from turnover, repairs between leases, or a slower rental season. The U.S. Census Bureau’s most recent housing data puts the national rental vacancy rate at 7.3%, meaning even in a healthy rental market, a meaningful share of rental units sit unoccupied at any given time. Using a conservative vacancy assumption, rather than assuming a tenant every single day of the year, keeps your numbers honest.

2. Calculate Every Expense Before You Buy

This is where most new investors get into trouble. It’s easy to remember big expenses like the mortgage and property taxes, and much easier to forget the smaller ones that add up fast. One common guideline, sometimes called the 50% rule, suggests that operating expenses on a rental property will typically eat up about half of the gross rental income, even before the mortgage payment. That’s a rule of thumb, not a guarantee, but it’s a helpful reminder that expenses are usually bigger than they first appear.

Operating Expenses to Include

• Property taxes
• Landlord insurance
• Property management
• Maintenance and repairs
• Vacancy
• HOA fees, if applicable
• Utilities paid by the owner
• Landscaping
• Licensing or permits, where applicable

Don’t Forget Big-Ticket Repairs

Beyond routine maintenance, plan for the big, expensive items that eventually need replacing: the roof, HVAC system, water heater, plumbing, appliances, and exterior repairs. These don’t happen every year, but when they do, they’re expensive. As one example, HVAC replacement alone averages around $7,500, and can run as high as $22,000 depending on the size of the home and system. If you don’t budget for these costs in advance, a single major repair can wipe out a year or more of cash flow.

From our own experience, the expenses we’ve learned not to underestimate are maintenance on older properties, the true cost of tenant turnover, and property management, even when we self-manage, since our own time still has a cost.

Keep a Reserve Cash Account

Budgeting for expenses on paper isn’t enough. You also need actual cash set aside to cover them when they happen, and they will happen at inconvenient times. A common starting point is three to six months of the property’s total operating expenses, including the mortgage, though we lean toward the higher end for older properties or ones with an aging roof or HVAC system. This account isn’t meant to sit untouched forever. It’s meant to be used, for a slow month of vacancy, a surprise repair, or a tenant who pays late. Without it, a single bad month can turn into a scramble to cover the mortgage out of your own pocket, which is exactly the kind of stress that leads people to sell a good property at the wrong time.

3. Calculate the Property’s True Cash Flow

Once you have realistic income and a complete list of expenses, the formula is simple:

Rental Income − Vacancy − Operating Expenses − Mortgage = Cash Flow

But here’s the part people miss: positive cash flow doesn’t automatically mean it’s a good investment. A property that only generates a few dollars a month in profit, after you’ve put tens of thousands of dollars into it, might be technically “cash flowing” while still being a poor use of your money.

Example: A $300,000 Rental Property

That property is technically cash-flow positive. But on a $70,000 total cash investment, including the down payment, closing costs, and initial repairs, $94 a month works out to a cash-on-cash return of only around 1.6%, well below the 8% to 12% range that’s often considered a solid return on a financed rental property. A related metric, the cap rate, measures a property’s return before financing is factored in, which makes it useful for comparing deals side by side regardless of how each one is financed. Looking at both numbers together gives a much clearer picture than cash flow alone.

4. Stress-Test the Deal Before You Buy

This step is what separates a careful investor from a hopeful one. Before buying, ask “what happens if”:

• Rent is 5% lower than expected?
• The property sits vacant for two months?
• Property taxes increase?
• Insurance costs rise?
• The HVAC needs replacing?
• You have a major repair in the first year?
• Property management costs more than expected?

The Deal Should Still Make Sense When Things Go Wrong

Don’t build your investment thesis around perfect occupancy and zero repairs, because that scenario doesn’t exist. Run a base case using your realistic numbers, then run a conservative case where several of the questions above go against you at once.

Going back to our $300,000 example: the base case showed a cash flow of $94 a month. Now stress-test it. Assume rent comes in 5% lower than expected, the property sits vacant for two months instead of a few scattered days, property taxes rise 5%, and insurance rises 10%. Run those same numbers again, and that property no longer earns $94 a month. It loses about $345 a month. That doesn’t necessarily mean the deal is dead, but it does mean the base case was hiding just how thin the margin really was. If the conservative case leaves you with a serious loss every month, the deal is riskier than it looks, and you should either renegotiate the price, find ways to increase income, or walk away.

5. Evaluate the Property, Not Just the Spreadsheet

Numbers matter, but they aren’t everything. A great spreadsheet won’t save you from a property with real, physical problems.

Check the Physical Condition

Look closely at the roof, HVAC, plumbing, electrical, foundation, appliances, and exterior. A home inspection before closing is one of the best investments you’ll ever make. It’s far cheaper to pay a few hundred dollars for a thorough inspection now than to discover a failing foundation or an aging roof after you’ve already closed. If the inspection turns up problems, you don’t have to walk away automatically, but you should use those findings to renegotiate the price or budget accordingly.

Research the Neighborhood

Look into rental demand, comparable rents nearby, local employment, schools and location factors, crime and neighborhood trends, and any future development that could affect the area. A property can pencil out perfectly on paper and still struggle to attract tenants if the surrounding area is declining, or if a wave of new construction is about to add a lot of competing rental supply nearby. Spend time actually visiting the neighborhood at different times of day, not just reviewing it online.

Estimate the Initial Cash Needed

Add up the down payment, closing costs, any needed repairs, initial cash reserves, and a buffer for unexpected expenses. This total, not just the purchase price, is what you’re really deciding to invest.

6. Our Rental Property Decision Checklist

Before we make an offer on any property, we run through this list:

• ☐ Realistic rent verified — We’ve checked actual comparable listings, not just the seller’s number, using at least two sources like Zillow, Rentometer, or a local property manager.
• ☐ Vacancy included — We’ve reduced expected income by a conservative vacancy percentage, rather than assuming the unit stays rented every single day of the year.
• ☐ Property taxes verified — We’ve pulled the actual current tax bill from the county assessor’s site, and checked whether taxes are likely to be reassessed higher after the sale closes.
• ☐ Insurance estimate obtained — We’ve gotten an actual quote from an insurance provider for landlord coverage on this specific property, not a rough guess based on a different property.
• ☐ Management included — Even if we plan to self-manage, we’ve budgeted a management-equivalent cost, since our own time isn’t free.
• ☐ Maintenance budget included — We’ve set aside a monthly amount for routine repairs, based on the property’s age and condition, not just an optimistic “nothing will break” assumption.
• ☐ CapEx reserve included — Separate from routine maintenance, we’ve budgeted for the eventual replacement of big-ticket items like the roof, HVAC, and water heater.
• ☐ Mortgage calculated — We’ve used the actual expected interest rate, loan term, and down payment, not a rough estimate from a different deal.
• ☐ Cash flow calculated — We’ve subtracted vacancy, every operating expense, and the mortgage from realistic rental income to get an actual monthly number.
• ☐ Cash-on-cash return calculated — We’ve measured that cash flow against the total cash we’re putting into the deal, including closing costs and repairs, not just the down payment.
• ☐ Property condition reviewed — We’ve had a professional inspection, or at minimum a careful walkthrough, covering the roof, HVAC, plumbing, electrical, and foundation.
• ☐ Neighborhood researched — We’ve looked into rental demand, comparable rents, employment, schools, crime trends, and any planned development nearby.
• ☐ Emergency reserves available — We have real cash set aside, outside of this deal, to cover a bad month or an unexpected repair without panic.
• ☐ Conservative scenario still works — We’ve stress-tested the deal against lower rent, added vacancy, and rising costs, and it still doesn’t lose serious money.

If the numbers only work under optimistic assumptions, we wouldn’t consider it a good investment, no matter how nice the property looks in photos.

When a Rental Property Isn’t a Good Investment

It’s just as important to know when to walk away. A good deal doesn’t have to be perfect, but the following warning signs mean the deal isn’t ready, or isn’t there at all.

Negative or extremely thin cash flow. A property that loses money every month, or one that only clears $20 or $30 after every expense is accounted for, has almost no room for error. One slow month of vacancy or one unexpected repair, and you’re paying out of pocket to own it.

Major deferred maintenance. If the roof, HVAC, plumbing, or foundation all need work soon, you’re not just buying the property, you’re buying a repair project. Those costs need to come out of the purchase price or your reserves, not out of hope.

Unrealistic rent assumptions. If the numbers only work assuming you can charge well above what comparable units in the area actually rent for, the deal doesn’t work. It just looks like it does until you try to find a tenant.

High property taxes or insurance costs relative to rent. In some markets, taxes and insurance alone can eat up a third or more of the rent before you’ve paid for anything else. Always check these two costs directly rather than estimating them, since they vary enormously by location and can quietly sink an otherwise decent-looking deal.

Weak rental demand in the area. A low price doesn’t matter if you can’t keep the unit occupied. Shrinking population, major employers leaving the area, or a glut of new rental construction nearby are all signs that demand may not support your income assumptions for long.

A deal that only works if the property appreciates significantly. Appreciation is a bonus, not a plan. If the cash flow and returns only look acceptable because you’re counting on the property being worth a lot more in a few years, you’re speculating, not investing. Prices can flatten or fall, and you may not get to choose when you sell.

Insufficient cash reserves. If buying the property would leave you with little or no cushion for repairs, vacancy, or a slow stretch, you’re one bad month away from real financial stress, even if the deal looks fine on paper.

Returns that aren’t attractive compared to other options for your money. A rental property that offers a 2% cash-on-cash return isn’t automatically a bad choice, but it’s worth asking honestly what else that same cash could earn with less time, effort, and risk. Real estate should be compared against your other options, not evaluated in isolation.

A rental property doesn’t have to be perfect, but the numbers need to hold up under realistic, even slightly pessimistic, assumptions. If they don’t, it’s not a good investment yet, and that’s a perfectly fine reason to pass and look at the next one.