Understanding cash flow in real estate: The formula and the mistakes

Written by Samantha SheltonNMLS 1647301Updated September 2026

This article explains an investing metric and is general information only. It is not tax, legal or investment advice, and not an offer to sell or a solicitation to buy any security. Take the tax math to a qualified tax professional and lease, title and entity questions to a Michigan attorney. Align Lending is an independent mortgage broker, not a broker-dealer, investment adviser, syndicator or property manager.

Cash flow in real estate is the money left after you subtract operating expenses and debt service from a property’s income, and the formula is one line: before-tax cash flow equals net operating income minus annual debt service. Fannie Mae’s own underwriting assumes a quarter of your rent never reaches you. Where a lender qualifies you on current lease agreements or the market rents reported on Form 1007 or Form 1025, the Selling Guide directs it to multiply gross monthly rent by 75%, because the remaining 25% is absorbed by vacancy losses and ongoing maintenance. Rental income supported by filed tax returns is calculated a different way.

That gap between the rent on the lease and the rent in your account is where most projections die. A property with $36,000 of net operating income and $27,000 of annual mortgage payments produces $9,000 a year, or $750 a month, before taxes. Whether that $750 is real depends on the assumptions underneath it.

Learn how cash flow in real estate works: how to build effective gross income, which costs count as operating expenses, why cash flow and profit are different numbers, and how cash flow drives cash-on-cash return. New to this? Start with our Real Estate Investing guide or the Investing Basics hub.

Key takeaways:

• Cash flow is NOI minus debt service: A property with $36,000 of net operating income and $27,000 of annual mortgage payments produces $9,000 a year, or $750 a month, before taxes.
• Conservative assumptions decide whether it is real: Fannie Mae has lenders count only 75% of gross rent from a lease or Form 1007 as qualifying rental income, and the national rental vacancy rate was 7.3% in the second quarter of 2026, so a projection built on 12 months of paid rent describes a market that does not exist.
• Cash flow and profit are not the same number: Principal repayment is cash out the door that you cannot deduct, and depreciation is a deduction with no cash attached, so a property can bank $750 a month and still report a small loss on your return.

What is cash flow in real estate?

Cash flow in real estate is the cash a property produces after every operating cost and every mortgage payment: before-tax cash flow equals NOI minus annual debt service. It answers the only question that matters month to month, whether the property pays you or you pay the property.

• Positive cash flow: Income exceeds operating expenses and debt service, so the property funds itself and pays you a surplus.
• Negative cash flow: The property costs more to hold than it produces, and you cover the gap yourself.
• It is a period measure, not a forecast: Cash flow describes twelve months of actual bills, not year five and not what the building is worth.
• It is personal to you: Change your down payment, rate or term and the same property produces a different number. NOI does not move; cash flow does.

That last point is why the return stack sits on top of net operating income: NOI is effective gross income minus operating expenses, measured before any mortgage payment, so it describes the building rather than the deal. Our explainer on what net operating income (NOI) is goes line by line.

How do you calculate cash flow on a rental property?

Five lines, and the second is where most projections break: gross potential income, minus vacancy, minus operating expenses, minus debt service, divided by 12.

1. Build gross potential income: Annual rent at full occupancy plus parking, laundry, storage, pet and application fees.
2. Subtract vacancy and credit loss: The line marketing sheets omit. The national rental vacancy rate was 7.3% in the second quarter of 2026 per the Census Bureau’s Housing Vacancies and Homeownership survey, unchanged from the first quarter. What’s left is effective gross income.
3. Subtract operating expenses: Taxes, insurance, repairs, management, owner-paid utilities, HOA dues, advertising, legal and accounting, turnover. The result is net operating income.
4. Subtract annual debt service: Twelve months of principal and interest. Both halves are cash out, though only the interest half is deductible.
5. Divide by 12: That’s monthly before-tax cash flow, the number to stress-test before you divide it by anything else.

Here’s the whole sequence on a hypothetical three-unit building bought for $450,000.

Watch the last two rows. A reserve is not an operating expense and does not reduce NOI, the IRS draws the same line, allowing a deduction for repairing or maintaining rental property but requiring you to capitalize any expense that betters, restores or adapts it. See the IRS list of deductible rental operating expenses. A roof is still cash leaving your account, so the $750 headline quietly becomes $500.¹ Run the sequence on a live listing in evaluating a property’s cash flow potential.

What is the difference between cash flow and profit?

Cash flow measures money moving; profit measures taxable income, and roughly $9,000 separates them on the same twelve months of the same building. The example property banks $9,000 of before-tax cash flow and reports close to break-even taxable income.²

• Principal repayment is cash out but not a deduction: Of $27,000 in payments, roughly $23,500 is deductible interest in year one and roughly $3,500 is principal. You cannot deduct mortgage principal payments as a rental expense, that $3,500 leaves your account and buys equity instead of a write-off.
• Depreciation is a deduction with no cash attached: Allocate 20% of a $450,000 purchase to land and the $360,000 building yields about $13,090 of annual depreciation that costs you nothing that year. It is not free. Basis falls by the depreciation allowed or allowable, so the bill arrives on sale. The recovery periods, the land-and-building basis split and the recapture math are in real estate depreciation.
• Together, the two diverge: $36,000 of NOI minus $23,500 of interest minus $13,090 of depreciation is a small paper loss while the account is up $9,000. Whether that loss reduces your tax bill depends on the passive activity loss rules, a question for your CPA, not a spreadsheet.

This is why “it lost money on paper” and “it paid me” are both true of one asset. Our overview of real estate tax benefits for investors is a starting point; a qualified tax professional is the finish line.

What is cash-on-cash return, and how does cash flow drive it?

Cash-on-cash return is annual before-tax cash flow divided by the total cash you invested, so cash flow is its numerator, which is exactly why every assumption that inflates cash flow inflates that return too. On this property, $9,000 of cash flow against $130,000 of cash invested is a 6.9% return, and the funded $6,000 is 4.6%.

The denominator, what counts as cash invested, what a good number looks like and what the measure leaves out are all handled in our guide to what cash-on-cash return is, which owns the metric. The point to carry back here is the direction of travel: fix the cash flow line and the return fixes itself; flatter the cash flow line and the return is fiction.

One neighboring metric is worth naming and setting aside: the cap rate is NOI divided by price ($36,000 on $450,000 is an 8.0% cap rate) and because it stops before debt service it prices the building rather than your deal, which our guide to cap rates in real estate covers in full. Run the three in order: NOI tells you what the building does, cash flow what you take home, cash-on-cash whether that was a good use of $130,000.

Why is my rental cash flow lower than I projected?

Almost always because the vacancy line was too kind. A common conservative assumption runs a vacancy allowance of roughly 5% to 8% of gross rent, and practitioners often assume expenses and vacancy together take about half of gross rent.³ Both are rules of thumb, not laws, test them against local conditions using rental market trends.

The other omissions that break a projection. Merging repairs with capital expenditures, inheriting the seller’s expense numbers, zeroing out management, skipping the financing quote. Are worked through line by line in evaluating a property’s cash flow potential, which owns the pro forma build.

Professional bodies such as IREM, the Institute of Real Estate Management, certify property managers and publish operations research precisely because these lines separate amateur underwriting from professional.

How does your financing change the cash flow?

Financing is the largest controllable line in the calculation, $27,000 of the $52,000 that comes in, or 52% of effective gross income. Change the loan and the answer changes without touching the building.

• Down payment and term: More cash down or a longer amortization lowers debt service and raises monthly cash flow, but more cash down also enlarges the cash-on-cash denominator. Higher cash flow and higher return are not the same goal. The tradeoff is the subject of real estate leverage.
• Pricing on investment loans: Investment purchases carry loan-level price adjustments plus higher down payment and reserve requirements that primary-residence loans don’t, and those land directly on cash flow. Shopping them is covered in financing a real estate investment.
• Debt service coverage: Lenders often divide NOI by annual debt service. Here that’s $36,000 divided by $27,000, or 1.33, operating income covers the mortgage 1.33 times over.
• Refinancing mid-hold: Resetting rate or term changes cash flow the month it closes. Run the break-even first, using how to refinance an investment property.

One financing point is not optional: representing an investment property as your primary residence to get better loan terms is occupancy fraud and a federal crime. The honest path is in buying a home as an investment property.

What Michigan investors should know

Two Michigan tax lines break more cash flow projections here than anything else, and both change the day you close. The seller’s tax bill will not warn you about either.

• You lose the principal residence exemption, worth up to 18 mills: The exemption covers an owner’s principal residence for up to 18 mills of local school operating millage, and the Michigan Department of Treasury states that vacation homes, seasonal homes and income property are not occupied as a principal residence and may not claim it. Your rental pays the non-homestead rate the owner-occupant next door does not.
• Taxable value uncaps the year after you buy: Under the Michigan Constitution as amended by Proposal A of 1994, a transfer of ownership uncaps taxable value in the calendar year following the transfer, and taxable value is then set equal to the state equalized value, which Michigan assesses at no more than 50% of true cash value. A long-time owner may be taxed on a fraction of what you will be.
• Underwrite winter as a line, not a surprise: Snow removal, heating vacant units against frozen pipes and freeze-related repairs are real Michigan operating expenses a summer walkthrough will not reveal.
• Michigan’s individual income tax rate is 4.25%: That is the statutory rate under MCL 206.51, subject to a conditional annual reduction that the state treasurer and the House and Senate fiscal agency directors determine each year by the date of the January revenue estimating conference, so confirm the rate for your tax year with Treasury or your CPA. It sits below cash flow entirely. Another reminder that before-tax cash flow is not take-home money.

Call the assessor for the state equalized value and the current non-homestead millage, then rebuild the tax line with those figures. This one correction moves more Michigan projections than anything else you can do. Take assessment appeals to a Michigan attorney and the tax math to a CPA; if you’re still deciding whether to buy, how to become a real estate investor covers the sequence.

FAQ: Cash flow in real estate

Here are answers to common questions about cash flow in real estate.

What is the 50% rule, and can I rely on it?

The 50% rule is a screening shortcut, not a law. It assumes operating expenses and vacancy together consume roughly half of gross rent, leaving the rest to cover debt service and whatever cash flow is left. It is useful for deciding which listings deserve an hour of work and useless for deciding whether to buy, because real expense ratios swing widely with building age, unit mix, who pays the utilities and whether management is actually paid. Build the lines instead, starting with analyzing a property’s rental potential.

What is the difference between NOI and cash flow?

NOI measures operating performance and excludes financing, so two investors buying the same building have the same NOI. Cash flow subtracts your specific debt service, so it reflects your loan amount, rate and term. NOI values the property; cash flow tells you what you take home. The full screening sequence is in how to spot a good real estate deal.

How much cash flow should a rental property produce?

There is no correct figure, and any per-unit target you have heard is a rule of thumb rather than a benchmark. What matters is whether the number survives full post-sale property taxes, real vacancy and funded maintenance and capital reserves. A deal that only works at zero vacancy is not a cash-flowing deal. Start with tips for new real estate investors.

Is negative cash flow ever acceptable?

Sometimes, but only as a funded decision rather than a surprise. Investors accept a negative number while they reset below-market rents or finish a renovation, and only when they can cover the monthly shortfall for years without selling. A deal that works only if values rise is a bet on the market, not a cash-flowing asset, and past performance does not predict future results. Size the shortfall in dollars a month before you sign, and read real estate market risk.

The bottom line on real estate cash flow

Cash flow in real estate is net operating income minus annual debt service, so $36,000 of NOI against $27,000 of mortgage payments produces $9,000 a year, or $750 a month, before taxes. Closer to $500 once a capital reserve is funded. It is the only metric in the stack that reflects your actual loan, it is the numerator in cash-on-cash return, and it differs from taxable profit because principal repayment is cash you cannot deduct while depreciation is a deduction with no cash attached.

If you’re ready to find out what a property’s cash flow looks like once your financing is actually priced, talk to Align Lending. We’ll shop your investment purchase or refinance across our lender network and show you the numbers side by side. Call 248-506-5727 or start online today. Take tax treatment to a qualified tax professional, entity and lease questions to a Michigan attorney, and any securities question to a licensed professional.

Footnotes:

1. Example figures are hypothetical and for educational purposes only; they do not constitute an advertisement of credit terms, a rate quote, an offer of credit, or a projection of investment returns under federal or state law. The $450,000 purchase price, $54,000 gross scheduled rent, $1,800 other income, $3,800 vacancy allowance, $52,000 effective gross income, $16,000 of itemized operating expenses, $36,000 net operating income, $337,500 loan amount, $27,000 annual debt service, $9,000 and $6,000 cash flow figures, $3,000 capital reserve, $130,000 total cash invested, and the resulting 6.9%, 4.6%, 8.0% and 1.33 figures are Align Lending arithmetic shown to demonstrate the formulas. No interest rate is stated or implied as available. They do not reflect any specific property or any specific loan, and actual results depend on the property, the market, your loan terms and your own underwriting. Return to text.

2. This is general information, not tax advice. The approximately $23,500 of first-year mortgage interest, $3,500 of first-year principal, 20% land allocation, $360,000 depreciable basis and approximately $13,090 of annual depreciation are illustrative arithmetic, not a computation of your tax liability. Depreciation recovery periods and conventions, land-and-building basis allocation, the deductible-versus-capitalized treatment of a given expense, passive activity loss rules, the qualified business income deduction and entity choice all depend on your individual circumstances and change over time. Consult a qualified tax professional about your situation. Align Lending is a mortgage broker and does not provide tax, legal or investment advice. Return to text.

3. The 5% to 8% vacancy allowance and the roughly 50% expense-and-vacancy figure are widely repeated practitioner rules of thumb, not statutory, regulatory or index-published standards, and they carry no predictive force. Vacancy, expense ratios and achievable rents vary by property type, submarket, unit mix, building age and management quality, and a property can fall well outside these ranges for sound reasons. Nothing here is a projection, guarantee or solicitation of any investment return, and past performance does not predict future results. Return to text.

Sources: the requirement that, when current lease agreements or the market rents reported on Form 1007 or Form 1025 are used, a lender calculate rental income by multiplying gross monthly rent by 75%, with the remaining 25% absorbed by vacancy losses and ongoing maintenance expenses, Fannie Mae Selling Guide B3-3.8-01, Rental Income (10/08/2025). National rental vacancy rate of 7.3% in the second quarter of 2026 and 7.3% in the first quarter of 2026, U.S. Census Bureau, Quarterly Residential Vacancies and Homeownership, Second Quarter 2026, released July 28, 2026. Deductibility of an expense for repairing or maintaining rental property, the requirement to capitalize any expense that betters, restores or adapts the property, the list of common deductible rental expenses, the rule that mortgage or principal payments may not be deducted as a rental expense, and the rule that depreciation reduces basis for figuring gain or loss on a later sale or exchange, Internal Revenue Service Publication 527, Residential Rental Property. Requirement to reduce the basis of property by the depreciation allowed or allowable, whichever is greater, with allowed meaning depreciation actually deducted and allowable meaning depreciation you were entitled to deduct, Internal Revenue Service Publication 946, How To Depreciate Property. Certification, education and research programs for property managers, Institute of Real Estate Management (IREM). Principal residence exemption from the local school operating millage of up to 18 mills, and the statement that vacation homes, seasonal homes and income property are not occupied as a principal residence and may not be claimed, Michigan Department of Treasury, Principal Residence Exemption, and MCL 211.7cc. Uncapping of taxable value in the calendar year following a transfer of ownership, with taxable value set equal to the state equalized value, Michigan Department of Treasury, Changes in Ownership and Uncapping of Property, and MCL 211.27a. State equalized valuation not to exceed 50% of true cash value, Michigan Constitution and Michigan Department of Treasury assessment guidance. Statutory Michigan individual income tax rate of 4.25% on and after October 1, 2012, and the conditional annual rate reduction to be determined by the state treasurer and the directors of the Senate and House fiscal agencies no later than the date of each January revenue estimating conference, MCL 206.51, Income Tax Act of 1967, Michigan Legislature. The 5% to 8% vacancy allowance and the 50% expense rule of thumb are drawn from general real estate investing literature and are presented as rules of thumb only.

This article is for informational purposes only and is not intended to provide legal, financial, tax or investment advice, and it is not an offer to sell or a solicitation of an offer to buy any security. Rents, vacancy, operating costs, tax rules, millage rates, assessments and loan program requirements change and vary by property, program and locality; confirm current figures with your loan officer, confirm tax treatment with a qualified tax professional, and confirm assessment, entity and lease questions with a Michigan attorney. Figures shown are illustrative, are not a rate quote or an offer of credit, and do not constitute an advertisement of credit terms or a projection of investment returns. Past performance does not predict future results. Align Lending is an independent Michigan mortgage broker, NMLS #2041154, and is not a broker-dealer, investment adviser, syndicator or property manager.

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