Cash flow, the honest version

Why rent minus the mortgage is a fantasy, and how to calculate the number that lands in your account.

The short version

Cash flow is what is left after every cost, not just the mortgage. The honest formula is: rental income, minus a vacancy allowance, minus all operating expenses (taxes, insurance, maintenance, management, and reserves for big repairs), minus the mortgage. What remains is what reaches your account.

The same rental with every cost included:

Line	Monthly amount
Rent collected	$2,000
Vacancy and credit loss	about $140
Operating expenses	about $800
Net operating income	about $1,060
Mortgage payment	about $1,300
Cash flow	about $240 short

Beginners compute rent minus mortgage and call the difference profit. That number is a fantasy, and chasing it is one of the most common ways landlords lose money. The discipline of the honest version is what separates a rental that builds wealth from one that quietly bleeds.

The fantasy math, and why it fails

It goes like this: rent is $2,000, the mortgage is $1,500, so I make $500 a month. Then the water heater fails. A tenant moves out and the unit sits empty for two months. The roof needs work in year eight. The $500 was never real, because it ignored costs that are certain to arrive, just not every month. Underestimating expenses is the single most common error in rental analysis, and it is what turns a deal that looks profitable on paper into a money pit.

What the honest version includes

Here is everything rent minus mortgage leaves out:

Vacancy. No rental stays full forever. Budget 5% to 10% of rent, more in weaker markets or older properties. Even 5% is about 18 days of empty unit a year.
Maintenance and repairs. The ongoing small fixes. Roughly 5% to 10% of rent, or about 1% of property value a year, as a starting point.
Capital expenditure reserves. The big replacements, roof, HVAC, water heater. You set this aside every month even when you do not spend it, because a $10,000 roof is a certainty on a long enough timeline. Another 5% to 10% of rent.
Property management. 8% to 10% of rent, even if you manage it yourself, because your time is not free and you may not always self-manage.
The obvious operating costs. Property taxes, insurance, any HOA dues, and any utilities you cover.

A quick sanity check is the 50% rule: across a portfolio, operating expenses (everything except the mortgage) tend to run about half of gross rent. If your analysis shows expenses far below that, you are probably missing something.

A worked example

Rent of $2,000 a month is $24,000 a year in gross income. Take out a 7% vacancy allowance, about $1,680, and effective income is $22,320. Now subtract operating expenses, the taxes, insurance, maintenance, management, and reserves above, at roughly 40% of gross rent, about $9,600. That leaves net operating income of about $12,720 a year, or $1,060 a month.

Finally, subtract the mortgage. At $1,300 a month in principal and interest, your real cash flow is $1,060 minus $1,300, which is negative $240 a month. The rent-minus-mortgage version said positive $700. The honest version says you are losing $240. Same property, opposite conclusion. That gap is the whole point.

The metric that keeps you honest

Monthly cash flow in dollars is useful, but to compare deals, use cash-on-cash return: annual cash flow divided by the total cash you invested (down payment, closing costs, and upfront repairs), times 100. It tells you the yield your own money earns, and many investors target 8% to 12%. Because it counts only the cash that left your pocket, it stays honest about financing.

This is the leveraged cousin of cap rate. Cap rate ignores your loan and measures the property on its own; cash-on-cash includes your loan and measures your actual return. Use them together. (See cap rate, explained properly for the unleveraged side.)

Equity is not cash flow

Paying down your loan builds equity, which is real wealth, but you cannot spend it without selling or refinancing. Do not count principal paydown as cash flow. The honest cash-flow number is spendable money, not net worth on paper, and confusing the two is how investors convince themselves a draining property is fine.

Do not trust the seller's pro forma

Listing numbers routinely overstate rent by 10% to 15% and understate expenses by 20% to 40%. Rebuild the analysis with your own conservative figures: real market rent at the low end, real tax and insurance quotes, and proper reserves. If a deal only works on the seller's pro forma, it probably does not work. One way some investors flip a thin or negative number positive is a dual-income or house-hack structure. (See the dual-income property for how that math changes.) And if you are weighing a short-term rental for higher income, remember the higher income comes with higher costs and more volatility, so the honest math matters even more. (See short-term rental pricing.)

What a good number looks like

A common benchmark is $100 to $200 per unit per month after everything, though the right target depends on your strategy. Some investors accept thin or negative early cash flow in high-appreciation markets, but only with the reserves to survive years of it, because appreciation is unreliable over any short window. Positive, honest cash flow is what makes a rental sustainable regardless of what prices do.

Frequently asked questions

How do I calculate rental cash flow?

Start with rental income, subtract a vacancy allowance, subtract all operating expenses (taxes, insurance, maintenance, management, and reserves for major repairs), then subtract the mortgage. What is left is your cash flow. The common mistake is computing only rent minus mortgage, which ignores costs that are certain to come.

Why is rent minus the mortgage not my real cash flow?

Because it leaves out vacancy, maintenance, reserves for big replacements like a roof or HVAC, and management. Those costs are real even though they do not hit every month. A property that looks like it makes $500 a month on rent minus mortgage can lose money once the honest costs are included.

What expenses do people forget?

Vacancy (budget 5% to 10% of rent), capital expenditure reserves for major replacements (another 5% to 10%), maintenance (about 1% of property value a year), and property management (8% to 10% of rent, even if you self-manage). A quick check is the 50% rule: operating expenses tend to run about half of gross rent.

What is a good cash-on-cash return?

Many investors target 8% to 12%. Cash-on-cash return is annual cash flow divided by the total cash you invested, including down payment, closing costs, and upfront repairs. Unlike cap rate, it accounts for your specific loan, so it reflects the actual yield on the money you put in.

Should I trust the income and expense numbers in a listing?

Be skeptical. Sellers' projected figures routinely overstate rent by 10% to 15% and understate expenses by 20% to 40%. Rebuild the analysis with your own conservative numbers: market rent at the low end, real tax and insurance quotes, and proper reserves. If the deal only works on the seller's pro forma, it probably does not work.

Sources: 2026 real estate investing guides and calculators on cash flow components, the 50% rule, vacancy and capital expenditure reserves, and cash-on-cash return benchmarks, plus widely cited findings that seller pro formas tend to overstate rent and understate expenses. Figures are rules of thumb; actual results vary by property and market.

This article is general information, not investment or financial advice. Run any property with your own due diligence and consult qualified professionals before investing.