Editor’s Pick
The real estate cash flow formula is simple: monthly cash flow equals total rental income minus operating expenses minus debt service. That’s what rental property cash flow means at its core, a single number that tells you whether a property pays you or costs you every month. Take a rental bringing in $2,000 in rent, with $600 in operating expenses and a $1,000 mortgage payment: $2,000 – $600 – $1,000 = $400 a month. Positive means the property hands you cash every month. Negative means you’re the one covering the shortfall.
The Real Estate Cash Flow Formula, Broken Into Its Three Inputs
Monthly cash flow comes from three numbers: total rental income, operating expenses, and debt service. Add the first, subtract the second and third, and what’s left is your cash flow.
Debt service is your mortgage payment, principal and interest only. It’s kept separate from operating expenses on purpose. Mixing the two either hides how much the property actually costs to run day to day, or double counts the mortgage across two different lines of the math. If your lender escrows property taxes and insurance into that mortgage payment, count them once, inside the mortgage line, not again as separate operating expenses further down. If they’re not escrowed and you pay them yourself, they belong in operating expenses instead, and debt service stays limited to principal and interest.
What Counts as Income
Rental income is your gross scheduled rent, plus every recurring add-on collected from residents (pet rent, parking, storage, laundry machines, and late fees), minus a vacancy allowance, before anything else touches the number.
Start with the full rent roll: base rent across every unit you own, plus any fee that shows up on a resident’s statement every month. A one-time charge, like a lease renewal fee, doesn’t belong here. A recurring one does.
Then apply a vacancy allowance, a percentage haircut for the months a unit sits empty between residents. Reserve 5-10% of gross rent for vacancy as a starting point, a range commonly used across the industry, then narrow it to your own property’s history and your local market once you have a full year of data behind you. A property in a tight market with long resident tenure can often run leaner than that. One in a high-turnover market should reserve more.
What Counts as an Expense (and Why the 50% Rule Exists)
Over a full year, operating expenses on a typical single-family rental run close to half of gross rent once vacancy, a CapEx reserve, and real maintenance are folded into the math alongside the obvious bills. That’s the entire premise behind the 50% rule: a quick sanity check, more estimate than guarantee, that assumes roughly half your rent goes to operating costs before the mortgage even enters the picture.
Operating expenses include:
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Property taxes
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Insurance
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Maintenance and repairs
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Property management fees, if you hire it out
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HOA dues, where they apply
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Utilities you cover instead of your residents
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Vacancy allowance
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A CapEx reserve
Two of those, vacancy and CapEx, are the line items tidy cash flow examples tend to skip, and they’re exactly where the honest number and the hopeful one part ways.
Capital expenditures (CapEx) cover the big, infrequent items: a roof, an HVAC system, a water heater, major appliances. These aren’t routine maintenance. They’re replacements that come due every 10 to 20 years depending on the component, and they cost thousands of dollars when they land. Budget for them as a monthly reserve instead of hoping the year they’re due isn’t this one. A commonly used starting point is 5-10% of gross rent set aside every month, though some investors prefer building the number component by component: estimate each major system’s remaining useful life and its replacement cost, then divide the cost by the years remaining and save that amount annually. Either method works. Skipping the line item entirely doesn’t.
For the full expense list with dollar ranges for each category, see our complete breakdown of rental property expenses.
A Worked Monthly Example, With CapEx and Vacancy Included
Add a CapEx reserve to the math, and this $2,000-a-month rental nets $80 a month, well short of the $240 you’d land on if you stopped one line short.
Here’s the property: a single-family rental renting for $2,000 a month, financed with a mortgage that runs $1,100 in principal and interest.
Income
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Gross rent: $2,000
Operating expenses
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Property taxes: $200
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Insurance: $100
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Maintenance and repairs: $100
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Property management (8% of rent, illustrative, actual fees commonly run 8-12%): $160
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Vacancy allowance (5% of rent, illustrative): $100
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CapEx reserve (8% of rent, illustrative): $160
Total operating expenses: $820
Debt service
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Mortgage (principal and interest): $1,100
Net cash flow: $2,000 – $820 – $1,100 = $80 a month
Drop the CapEx line and everything else stays the same. This property clears $240 a month instead, the number a lot of worked examples stop at. That single line item, easy to skip because it isn’t due this month, cuts the real margin by two thirds. It’s still due eventually. Budgeting for it now is what keeps a good-looking spreadsheet from becoming a bad month three years from now when the roof actually fails.
Positive vs. Negative Cash Flow: What the Number Is Actually Telling You
A positive number is cash the property pays you after every bill is covered. A negative number means you’re the one funding the shortfall, out of your own pocket, every single month.
Positive cash flow doesn’t mean a property is problem-free. Negative cash flow doesn’t automatically mean walk away either. Some investors accept a temporary negative on a property they expect to appreciate. But for most rental investors, the number is the plainest signal you have of whether a property is working.
Three things tend to flip a property from positive to negative:
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Rising vacancy. A market that goes from 5% vacant to 12% vacant erases a chunk of gross income before a single expense line moves.
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Climbing maintenance. An aging roof, HVAC system, or plumbing stack that starts needing serious, non-routine repairs turns a modest cushion into a monthly loss fast.
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Tax and insurance jumps. Reassessments and rising premiums are two of the least controllable costs on the expense side, and both have been climbing faster than rent in a lot of markets.
Watch those three, and re-run the formula whenever one of them moves. A property that cash flowed $200 a month at purchase can go negative within a couple of years if you never check the math again.
Cash Flow Is One Number, Not the Whole Analysis: Pair It With GRM Before You Buy
Cash flow measures a property you’ve already modeled line by line: income, expenses, debt service, vacancy, and a CapEx reserve, all accounted for. The gross rent multiplier (GRM) does the opposite job. It’s the fast first-screen ratio for deciding which properties are even worth modeling that closely.
GRM is a property’s price divided by its gross annual rent. Run it across ten listings in an afternoon, rank them, and you’ve narrowed a long list down to the handful actually worth a full cash flow calculation.
If you’re new to rental investing and still deciding whether to buy at all, investing in rental property for beginners is the place to start before you run either number. And if the property in front of you is a duplex, triplex, or larger, evaluating a multifamily investment property covers the added math a multi-unit deal requires, on top of everything here.
FAQ
What is rental property cash flow?
Rental property cash flow is the money left over each month after you subtract operating expenses and debt service from total rental income. A positive number pays you. A negative number means you’re covering the gap yourself.
What income counts toward cash flow?
Gross scheduled rent counts, plus every recurring fee tied to the property: pet rent, parking, storage, laundry machines, and late fees. One-time charges don’t count. A vacancy allowance comes out of this total before you move to expenses.
What expenses get deducted?
Property taxes, insurance, maintenance and repairs, property management fees, HOA dues, utilities you cover instead of your residents, a vacancy allowance, and a CapEx reserve for major replacements all get deducted. The mortgage payment is deducted separately, as debt service, its own line apart from operating expenses.
Should you budget for vacancy?
Yes. A commonly used starting range is 5-10% of gross rent set aside for vacancy, adjusted to your property’s actual history and your local market once you have a year of data behind you. Skipping this line is one of the fastest ways to overstate how much a property really nets.
Why does cash flow matter?
Cash flow is the clearest signal you have of whether a rental is actually working. Strong appreciation or a great location can’t cover a property that loses money every month indefinitely, and cash flow is the number that tells you, in real time, which side of that line you’re on.
This article is general information, not legal advice. Laws change, and every situation is different. For guidance on a specific situation, talk to a qualified financial or tax professional.
