The Math Everyone Gets Wrong
Ask a landlord how much their rental makes, and most will give you a version of this answer: "Rent is $2,200. Mortgage is $1,400. So I'm making $800 a month."
That number is wrong. Not slightly off — structurally wrong. It's like calculating your personal savings as salary minus rent and ignoring groceries, insurance, car payments, and the fact that your roof needs replacing in five years.
The actual cash flow on most rental properties is 30-60% lower than the "rent minus mortgage" number landlords carry in their heads. And the gap between the felt number and the real number is where portfolios quietly bleed.
There is one number that tells you whether your rental is actually working. Most landlords can't name it about their own property.
The Real Calculation
Actual cash flow starts at the top and subtracts everything — not just the mortgage. Here's the full waterfall:
Step 1: Gross Rental Income
Total rent collected over the period. Not the asking rent — the actual collected rent. If you charged $2,200/month but had one month of vacancy, your annual gross is $24,200, not $26,400. Include any additional income: late fees, pet fees, laundry, parking. Start with reality.
Step 2: Subtract Vacancy Allowance
Even if your unit has been occupied for three straight years, you need to account for future vacancy. Industry standard: 5-8% of gross income. On a $2,200/month unit, that's $110-$176/month set aside. You won't spend it every month, but when vacancy hits, you'll need it.
If you skip this step, your cash flow looks great until the day a tenant leaves — and then one month of vacancy wipes out four months of "profit."
Step 3: Subtract Operating Expenses
This is where the rent-minus-mortgage calculation falls apart. Operating expenses include:
Property taxes. Often higher than they need to be and rising annually.
Insurance. Premiums are up 19% in recent years while rent growth has been 3-5%.
Maintenance and repairs. Budget 1-2% of property value annually. A $300,000 property should earmark $3,000-$6,000/year for maintenance.
Management costs. If you use a PM, that's 8-10% of gross rent. If you self-manage, your time still has a cost — even if you choose not to count it, the hours are real.
Utilities (if landlord-paid). Water, sewer, trash, common area electric.
HOA or condo fees (if applicable).
Landscaping, snow removal, pest control — the recurring costs that add up quietly.
Add them all up. Gross income minus vacancy allowance minus operating expenses gives you your Net Operating Income (NOI).
Why NOI Matters
NOI is your property's operating profit before financing costs. It tells you whether the property works as a business, regardless of how you financed it.
Institutional investors obsess over NOI because it's the purest measure of property performance. If a property has a strong NOI, the financing is a separate optimization. If the NOI is weak, no amount of clever financing fixes the underlying problem.
Most small landlords can't tell you their NOI. If you're serious about managing your properties like assets, this is the first number to learn.
Step 4: Subtract Debt Service
Now subtract your mortgage payment (principal and interest). This is the number most landlords start with. In the real calculation, it comes fourth.
Note that the principal portion of your payment is technically building equity, not disappearing. Some landlords exclude it from the cash flow calculation for that reason. But it's still cash leaving your bank account each month, so for cash flow purposes — money available to you — it's a subtraction.
Step 5: Subtract Capital Expenditure Reserves
Your roof has a 25-year life. Your HVAC system has a 15-year life. Your water heater has a 10-year life. These replacements are not surprises — they're inevitable expenses with known timelines.
A CapEx reserve sets aside 5-10% of gross income monthly to fund these inevitable replacements. On a $2,200/month unit, that's $110-$220/month.
Skip this and your "cash flow" looks great for 9 years. Then the HVAC dies and you need $8,000 you don't have. That's not a surprise expense. That's a budgeting failure.
Step 6: What's Left Is Actual Cash Flow
Gross income, minus vacancy allowance, minus operating expenses, minus debt service, minus CapEx reserves. What remains is your actual, spendable cash flow.
Running the Numbers on a Real Property
Let's put this together. Take a single-family rental with $2,200/month in rent and a $1,400/month mortgage payment (the one where the landlord "makes $800/month").
Gross annual income: $26,400
Vacancy allowance (5%): -$1,320
Property taxes: -$3,600
Insurance: -$1,800
Maintenance/repairs: -$3,000
Landscaping/pest/misc: -$1,200
NOI: $15,480
Debt service (mortgage): -$16,800
CapEx reserves (7%): -$1,848
Actual annual cash flow: -$3,168
That $800/month "profit" is actually a $264/month loss when you count everything.
This doesn't mean the property is a bad investment — equity buildup, appreciation, and tax benefits (including depreciation) may justify holding it. But the cash flow is negative. The landlord who thinks they're making $800/month is unknowingly subsidizing the property from other income.
The Benchmarks That Matter
Once you know your actual cash flow per door, you need context. How does your number compare?
$100-$200 per door per month after all expenses and reserves: You're doing fine. The property is self-sustaining with a modest return. Most rental properties in expensive markets fall in this range.
$200-$400 per door per month: Strong. You've either acquired well, managed expenses tightly, or both. This is the sweet spot for long-term holds.
$400+ per door per month: Excellent. Common in lower-cost markets or with properties that have been held long enough for mortgage paydown and rent increases to create significant margin.
Under $100 per door per month: Investigate. You're one maintenance surprise away from negative cash flow. Check your expense categories — is insurance too high? Are you overpaying vendors? Is the rent below market?
Negative cash flow: Not automatically a problem if you're holding for appreciation or tax benefits. But you need to know it's negative, and you need to know why.
The Operating Expense Ratio
Another useful benchmark: your operating expense ratio — total operating expenses divided by gross rental income.
For residential rentals, a healthy ratio is 35-45%. If your ratio is above 50%, something is likely out of line: vendor costs are too high, property taxes need an appeal, insurance hasn't been shopped in years, or the property needs more maintenance than it should.
The ratio gives you a quick diagnostic. If it's in range, your expense structure is probably fine. If it's high, you know where to start digging.
Three Levers When the Number Is Wrong
If your actual cash flow is lower than it should be, you have exactly three levers:
Increase revenue. Raise rents to market. Add fee income (pet rent, parking, storage). Consider ratio utility billing where applicable. But be realistic — aggressive rent increases carry turnover risk.
Decrease expenses. Audit vendor costs. Shop insurance. Appeal property taxes. Review escrow for overcharges. This is often the fastest path to improvement because you control it directly.
Restructure financing. Refinancing at a lower rate directly reduces debt service. But at today's rates, this only works if your current rate is significantly above market. Check the break-even math before committing.
Know Your Number
The difference between a landlord who is building wealth and one who is slowly bleeding money is not luck, location, or leverage. It's whether they know their actual cash flow per door — the real number, not the comfortable fiction of rent minus mortgage.
Calculate it for every property you own. Update it quarterly. Compare it to the benchmarks. When it drops, diagnose why. When it's strong, understand what's working so you can replicate it.
Everything else in rental investing — scaling, optimizing, deciding what to buy and what to sell — depends on this one number being accurate.
If you don't know your actual cash flow per door, you don't know if your rental is working. And you can't fix what you can't see.