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Rent minus mortgage is not cash flow. Real cash flow accounts for vacancy, every operating cost, and the full debt payment, and at today's financing costs, a lot of deals that looked good on a napkin do not clear the bar.
By RentalAnalytics Editorial Team · Published August 5, 2026
Cash flow is what is left over each month after collecting rent and paying every real cash cost of owning the property, including the mortgage. The formula is effective gross income minus operating expenses minus debt service. Each term matters: effective gross income already accounts for vacancy, operating expenses cover the recurring cost of running the property, and debt service is the full loan payment, not just interest.
Landlords who skip straight from rent to "cash flow" without running all three steps routinely overstate what a property actually produces. A unit that rents for $2,400 a month is not generating $2,400 a month in anything usable until vacancy, expenses, and the mortgage payment are all subtracted.
NOI is effective gross income minus operating expenses, calculated before any mortgage payment. It measures what the property itself earns, independent of how it is financed, which is why lenders and appraisers use NOI, not cash flow, to compare properties and size loans. Two investors with different down payments on the same property get different cash flow numbers but the same NOI.
Effective gross income starts with scheduled rent, then subtracts a vacancy allowance. A property renting for $2,400 a month with a 7% vacancy assumption produces $26,784 in effective gross income for the year, not the full $28,800 a fully occupied unit would collect.
Operating expenses are the recurring costs of running the property: property taxes, insurance, maintenance and repairs, property management fees, and any utilities the landlord covers. What does not belong in this bucket: mortgage principal and interest (that is debt service, a separate line), capital improvements like a new roof (that is capex, not an operating cost), and depreciation (a tax accounting entry, not a cash cost).
The Census Bureau and HUD jointly publish the Rental Housing Finance Survey, described in the agencies' own words as a survey that "provides a snapshot of the financial health of single-family and multifamily rental housing."[1] The most recent release found mean total operating expenses of $6,194 per year for one-unit rental properties in 2020, about $516 a month, versus $4,935 for 2-to-4-unit properties.[1] That gap is a reminder that per-unit costs do not scale down cleanly as building size grows.
More than most first-time landlords expect. Debt service, the full principal and interest payment, is usually the single largest line item in the entire calculation, larger than taxes, insurance, and maintenance combined. Small changes in the interest rate or the down payment percentage swing monthly cash flow by triple digits on an ordinary single-family rental.
Take a $320,000 property renting for $2,400 a month, 7% vacancy, $4,200 in annual property tax, $1,600 in insurance, $150 a month in maintenance, and an 8% property management fee. NOI comes out to $17,041 a year regardless of financing. What happens next depends entirely on the loan:
| Scenario | Down payment | Rate | Monthly cash flow | Cash-on-cash |
|---|---|---|---|---|
| Current market | 25% ($80,000) | 6.66%[2] | -$122 | -1.6% |
| Larger down payment | 35% ($112,000) | 6.66%[2] | +$83 | 0.8% |
| Lower historical rate | 25% ($80,000) | 5.5% | +$57 | 0.8% |
The 30-year fixed mortgage rate averaged 6.66% for the week ending July 30, 2026, according to Freddie Mac's Primary Mortgage Market Survey.[2] At that rate and a standard 25% down payment, the property above runs a small monthly loss before ever touching reserves for capital repairs. The same property turns modestly positive with either a larger down payment or a lower rate. Rent and expenses never change across the three scenarios. Financing alone decides whether the deal works.
Cash-on-cash return is annual pretax cash flow divided by total cash invested, expressed as a percentage. Total cash invested means the down payment plus closing costs and any immediate repair spend, not the full purchase price. On the current-market scenario above, $80,000 down plus roughly $9,600 in closing costs puts total cash invested near $89,600, so a $122 monthly loss produces a cash-on-cash return of about -1.6%.
Cash-on-cash is the return metric that reflects leverage. Two identical $320,000 properties financed differently, one at 25% down and one at 35% down, produce different cash-on-cash figures even though they generate the same NOI, because the smaller down payment puts less of the buyer's own money at risk for the same underlying property.
There is no universal answer, but a common investor convention targets 8 to 12% cash-on-cash and roughly $100 to $200 of positive monthly cash flow per unit as a starting screen. Treat that as a convention, not a citable benchmark: the right threshold depends on the market, the property's risk profile, and what else the cash could otherwise be doing. A property that clears NOI comfortably but runs thin cash flow purely because of an aggressive loan is a financing problem, not a property problem, and refinancing or a larger down payment can fix it without touching the rent roll.
Run your own numbers rather than relying on a rule of thumb from a different market and interest rate environment. The RentalAnalytics cash flow analyzer takes the same purchase price, financing, income, and expense inputs used in the walkthrough above and returns a full monthly and annual P&L, cash-on-cash, and break-even occupancy in one pass.
Cash flow equals effective gross income minus operating expenses minus annual debt service. Effective gross income is scheduled rent after subtracting a vacancy allowance. Operating expenses cover taxes, insurance, maintenance, and management. Debt service is the full mortgage payment, principal and interest included.
Net operating income (NOI) is effective gross income minus operating expenses, before any mortgage payment. Cash flow is NOI minus debt service. NOI measures a property's earning power independent of financing, so it is what lenders and appraisers use to compare deals.
Many investors treat 8 to 12% cash-on-cash as a solid target, though this is a convention, not a rule. The right threshold depends on your market, risk tolerance, and alternative uses for the cash. A property below your target can still make sense for appreciation or tax reasons.
The Census Bureau and HUD Rental Housing Finance Survey found mean total operating expenses of $6,194 per year for one-unit rental properties in 2020, about $516 a month, excluding debt service and capital improvements. Use this as a sanity check against your own itemized estimate.
Significantly. On a $320,000 property with 25% down, moving the rate from 5.5% to the 6.66% average reported by Freddie Mac in late July 2026 turns roughly $57 a month in cash flow into a $122 monthly loss, with rent and expenses held constant.
Before. Vacancy loss is subtracted from scheduled rent first to produce effective gross income. Operating expenses and debt service are then subtracted from that effective gross income figure, not from the full scheduled rent, since a vacant unit was never collected in the first place.
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