Cash flow is the only metric in rental property investing that cannot be faked. Appreciation is speculative. Tax benefits depend on your personal situation. Cash flow is what lands in your account every month after every bill is paid.
Most investors calculate it wrong. Not because the math is hard - it is not - but because they leave out the expenses that do not show up every month.
The formula
Cash flow equals gross rental income minus all operating expenses minus mortgage payment (debt service).
That sounds simple. The work is in the "all operating expenses" part.
A complete expense calculation for a single-family rental includes:
- Property taxes (divide the annual bill by 12)
- Insurance (landlord policy, not homeowner)
- Property management fee (typically 8-12% of gross rent)
- Vacancy allowance (industry standard is 5-8% of gross rent)
- Maintenance and repairs (budget 1% of property value per year as a baseline)
- CapEx reserve (set aside 5-10% of rent for large future expenses - roof, HVAC, appliances)
- HOA fees where applicable
- Lawn care and snow removal where applicable
Investors who exclude vacancy allowance and maintenance reserves are not calculating cash flow. They are calculating best-case scenario income. Those two line items alone account for 15-20% of gross rent in a realistic projection.
A worked example
Property: 3-bed single-family rental in Columbus, Ohio Purchase price: $220,000 Down payment: 25% ($55,000) Loan amount: $165,000 at 7.2% over 30 years Monthly mortgage payment: $1,121
Monthly gross rent: $1,650
Expenses: - Property taxes: $225/month - Insurance: $80/month - Property management (10%): $165/month - Vacancy allowance (6%): $99/month - Maintenance reserve (1% of value / 12): $183/month - CapEx reserve (7%): $116/month
Total monthly expenses: $868 Monthly mortgage: $1,121 Total outgoings: $1,989
Monthly cash flow: $1,650 - $1,989 = -$339
This property does not cash flow at current prices and rates in this market. That is not a hypothetical - it is the reality for a large portion of single-family rentals acquired in 2024 and 2025 in mid-tier markets. FRED housing data shows investor purchase activity has pulled back significantly in markets where this math stopped working.
What makes a property cash flow positive
Three levers: purchase price, rent level, and financing terms.
Purchase price below market. A property purchased at $180,000 in the same example changes the monthly mortgage to $917, turning the deal cash flow positive at around $130/month. Distressed properties, off-market deals, and estate sales are where the margin lives.
Higher rent-to-value ratio. The 1% rule (monthly rent equals 1% of purchase price) is a rough screening tool, not a guarantee. A $200,000 property renting for $2,000/month is roughly breakeven at current rates with a 25% down payment. Below 0.8%, cash flow is unlikely without a significant equity position.
Cash purchases or lower rates. Eliminating or reducing debt service changes the math dramatically. Investors who purchased with cash or locked in sub-4% rates in 2020-2021 have a structural advantage over anyone entering the market today. According to Bankrate's mortgage data, the monthly difference between a 4% and 7% loan on $165,000 is roughly $300 - which is the difference between positive and negative cash flow in many scenarios.
Cash-on-cash return
Cash flow is absolute. Cash-on-cash return is relative to what you invested.
Cash-on-cash = annual cash flow / total cash invested
If you put $55,000 into the deal and generate $3,600 in annual cash flow ($300/month), your cash-on-cash return is 6.5%. That is a reasonable return for a stabilized rental in a strong rental market. It is not an exceptional return.
Experienced investors generally target 8-12% cash-on-cash for value-add deals and accept 5-8% for stabilized properties in high-appreciation markets where they expect equity growth to supplement income.
Net operating income and cap rate
NOI (net operating income) is gross income minus operating expenses, before debt service. It is the number appraisers and commercial lenders use because it separates property performance from financing structure.
Cap rate = NOI / property value
A property generating $14,400 in annual NOI valued at $220,000 has a cap rate of 6.5%. Investopedia's real estate metrics guide explains cap rate in detail, including its limitations as a standalone metric.
Cap rates vary significantly by market and property type. Single-family rentals in primary markets trade at 4-5% cap rates. Secondary and tertiary markets offer 6-8%. Anything above 8% usually has a story attached to it.
This article is for informational purposes only and does not constitute financial, legal, or tax advice. Consult a qualified professional before making investment decisions.
