How to Estimate Rental Property Cash Flow (and What to Do When It's Negative)
Nobody teaches you how to estimate rental property cash flow when you never planned on being a landlord.
Maybe you inherited the house. Maybe work moved you to another city before your home sold. Maybe you listed it, watched it sit, and realized the offers coming in would cost you real money at the closing table. So you rented it out.
If that sounds like you, you have plenty of company. Zillow research from late 2025 found that 2.3% of homes listed for rent nationwide had recently been listed for sale, one of the highest shares since Zillow started tracking it. San Antonio ranked fourth in the country at 3.9%.
Here's the part most articles won't say out loud: a lot of these owners are losing money every month. Rents in San Antonio have softened, homes are taking longer to lease, and property taxes and insurance keep climbing. For many owners, the rent simply does not cover the mortgage.
This guide walks through how to estimate your rental property cash flow, what negative cash flow really means for you, and your honest options for fixing it or getting out.
Table of Contents
Why So Many San Antonio Owners Are Cash Flow Negative Right Now
A few forces stacked up at the same time:
Interest rates are high while inventory is also high. Those two usually move in opposite directions. Right now owners face both, so selling often means accepting less than they hoped.
Property taxes are heavy. Texas has no state income tax, so counties lean on property taxes instead. If you moved out and lost your homestead exemption, your tax bill likely jumped.
Insurance costs have surged. Premiums have climbed sharply, and some owners struggle to find coverage at all.
Rents have softened. More homes are competing for tenants, days on market have stretched out, and asking rents have come down in many neighborhoods.
Put those together and you get an owner who can't sell without a painful loss and can't rent for enough to cover the payment. That's the squeeze. Naming it clearly is the first step to managing it.
How to Estimate Your Rental Property Cash Flow
Cash flow is what's left after everything gets paid. The formula is simple:
Monthly cash flow = monthly rent minus all monthly expenses
The mistake most accidental landlords make is counting only the mortgage. Here's the full list to include:
Mortgage payment (principal, interest, taxes, insurance)
HOA dues, if any
Property management fees
Maintenance reserve
Vacancy reserve
Any utilities or lawn care you cover
Leasing fees and make-ready costs, spread across the year
Here's a hypothetical example with rounded numbers, just to show how the math works:
| Item | Monthly |
|---|---|
| Rent | $1,900 |
| Mortgage (PITI) | −$1,750 |
| Management fee | −$190 |
| Maintenance reserve | −$160 |
| Vacancy reserve | −$100 |
| Cash flow | −$300 |
On paper this owner "covers the mortgage." In reality, they're paying about $300 a month to hold the property. That gap is why so many owners feel broke even when the rent check clears.
If you want to track this properly, a simple spreadsheet with one row per month and one column per expense will tell you more than any gut feeling.
What Negative Cash Flow Actually Means
Negative cash flow means you're subsidizing the property out of pocket each month. That's a real cost and you should treat it like one.
It's also only part of the picture. While you hold the property, three other things are happening:
Your tenant is paying down your loan. Part of every mortgage payment goes to principal. In the example above, if $400 of the payment goes to principal, the owner's net worth grows even in a month where their checking account shrinks.
The tax code works in your favor. Rental expenses like mortgage interest, property taxes, insurance, management fees, and repairs are generally deductible against rental income on Schedule E. You can also depreciate the building over 27.5 years, which often creates a paper loss that softens the tax bill. Talk to a CPA about your situation, because income limits and passive loss rules apply.
You keep your option to sell later. If you'd lose $25,000 selling today, renting at a $300 monthly loss costs $3,600 a year. That buys you time for equity to recover without locking in the loss.
None of this makes negative cash flow fine. It just means the sell-or-hold decision needs real math, which we'll get to below.
Price It Right From Day One
If you take one thing from this article, take this: overpricing your rental to close a cash flow gap usually makes the gap worse.
Run the vacancy math. Say your home should rent for $1,900 based on what similar homes have actually leased for. You list at $2,000 hoping to shrink your monthly loss. Every month it sits vacant costs you $1,900 in lost rent plus the full mortgage you're covering alone. If it takes two extra months to lease at the higher price, you're out roughly $3,800. Earning that back at $100 extra per month takes over three years, longer than most tenants stay.
In a market with record inventory and falling rents, tenants have choices. Homes priced at the market lease. Homes priced above it sit.
A few practical rules:
Price against homes that actually leased, not homes still sitting on the market. Asking prices in a soft market are wish lists.
Watch your first two weeks. Showings but no applications usually means the price is off.
Small price cuts early beat big price cuts late. A $50 drop in week two costs far less than a month of vacancy.
Our Vacancy Cost Calculator adds up what an empty month costs on your specific property and lets you test whether a lower rent that leases faster comes out ahead. And if you're not sure what your home would lease for, start with a free rental analysis.
Cut the Monthly Bleed
You have more control over expenses than most owners realize:
Protest your property taxes every spring. In Texas you can protest your appraised value each year with your county appraisal district. Many owners never do, and assessed values in recent years have given owners plenty to work with. Bexar County recently passed an ordinance that affords an owner a tax freeze if they successfully appeal their county appraised property tax value.
Shop your insurance at every renewal. Landlord policies vary widely in price. An independent agent can quote several carriers at once. Also confirm you have a landlord policy rather than a homeowner policy, since the wrong policy can leave you uncovered.
Spend small on prevention. A $150 HVAC tune-up beats a $6,000 emergency replacement in July, which is why we require tenants to enroll in our Resident Benefits Package. Filters are automatically delivered to their door as a reminder to help maintain the HVAC system.. Changing filters, flushing the water heater, and checking for slow leaks keeps small problems small.
Fight to keep good tenants. Turnover is one of the biggest hidden costs in rentals. Between vacancy, make-ready, and leasing fees, one turnover can erase a year of thin margins. A modest renewal incentive is usually cheaper than a new tenant search.
Forecast Maintenance So It Stops Surprising You
Surprise repairs wreck more rental budgets than anything else. You can't predict the exact month a water heater dies, but you can budget so it doesn't hurt.
Three common rules of thumb:
The 1% rule. Budget about 1% of the property's value per year for maintenance. A $300,000 home means about $3,000 a year, or $250 a month.
The square footage rule. Budget about $1 per square foot per year. A 2,000 square foot home means about $2,000 a year.
The 50% rule. Over the long run, expect all operating costs combined (maintenance, taxes, insurance, management, vacancy) to eat roughly half your rent.
These are estimates, and older homes run higher. Sharpen your forecast by checking the age of the big-ticket systems. As general ranges, an HVAC system often lasts 10 to 15 years, a water heater 8 to 12, and a roof 15 to 30 depending on material and hail history. If your AC is 12 years old in San Antonio heat, the replacement isn't an if. Put it in the budget now.
Then open a separate savings account for the property and move your reserve into it every month. When the repair comes, it's an inconvenience instead of a crisis.
Run the Sell vs Hold Math Once a Year
Here's the frank math promised at the top. Once a year, put two numbers side by side: what selling now would cost you at the closing table, and what holding costs you per year after the principal your tenant pays down. If selling today locks in a $25,000 loss and holding runs $3,600 a year while your loan balance drops by $5,000 a year, holding looks reasonable as long as you have reserves to ride it out. If the monthly gap is $800, the house needs a roof, and you have no cushion, taking the loss can honestly be the cheaper path.
You don't have to build a spreadsheet for this. Our Rent vs Sell Calculator runs the full comparison with your numbers, including your loan paydown and a future sale, and shows which path comes out ahead. Owners who avoid this math usually end up selling anyway, just later and more tired.
FAQ
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Add up every monthly cost: mortgage with taxes and insurance, HOA, management, a maintenance reserve, and a vacancy reserve. Subtract that total from the monthly rent. If you skip the reserves, you're estimating hope instead of cash flow.
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No. It's a cost you should measure and plan for. Principal paydown, tax deductions, and the chance to sell in a better market can outweigh a small monthly gap. A large gap with no reserves and no plan is the dangerous version.
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Compare the loss you'd take selling today against your yearly cash flow gap minus yearly principal paydown. Reserves and stress matter too. Our Rent vs Sell Calculator runs this comparison, and it's worth rerunning annually because the answer changes as the market does.
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Common rules of thumb are 1% of the property value per year or $1 per square foot per year, adjusted up for older homes and aging systems.
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What similar homes have actually leased for recently, not what other listings are asking. In a soft market, pricing at the market and leasing fast almost always beats holding out for a higher number. Our free rental analysis pulls comparable leases from your neighborhood.
Get a Straight Answer on Your Numbers
Our team manages rental homes across San Antonio, and we watch this market every week. If you want a no-pressure look at what your home would actually rent for, what it costs to hold, and whether renting or selling makes more sense for you, reach out. We'd rather give you an honest answer now than manage a property that's quietly draining you.
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