Rental Property Cash Flow: The Only Number That Matters

Ask a new real estate investor why they're buying a rental property and you'll usually hear something about appreciation. "Property values always go up." "I'll build equity over time." "It's a long-term wealth play."

Sure. But appreciation is a guess about the future. Cash flow is money in your account this month. One of those things pays your bills. The other one looks good on a spreadsheet.

A Property That Looks Great on Paper

Let's walk through a real example. Here's a duplex listed at $280,000 in a mid-sized city. Each unit rents for $1,200/month. Gross rental income: $2,400/month, or $28,800/year.

You put 25% down ($70,000) and finance the rest at 7.25% on a 30-year loan. Your monthly mortgage payment (principal and interest) is $1,432.

$2,400 rent minus $1,432 mortgage = $968/month. Almost a thousand bucks a month in cash flow, right?

Not even close.

Where the Money Actually Goes

Here's what that $2,400/month really looks like once you account for actual operating costs:

Property taxes: $375/month. This varies wildly by location, and it often increases after a property changes hands. Don't use the seller's tax bill as your number – check what the assessment will reset to after purchase.

Insurance: $165/month. Landlord policies cost more than homeowner policies. If the property is in a flood zone, add more.

Vacancy: $192/month (8% of gross rent). You won't have both units rented every day of every year. Turnovers happen. Sometimes a unit sits empty for a month between tenants. Budgeting 8% accounts for roughly one month of vacancy per unit every two years.

Maintenance: $240/month (10% of gross rent). Two units means two sets of everything that can break. Water heaters, garbage disposals, faucets, doors, locks. Stuff breaks constantly.

CapEx reserves: $192/month (8% of gross rent). The roof will need replacing eventually. So will the furnace, the water heaters, the windows. Setting money aside monthly means a $12,000 roof replacement in year 9 doesn't come out of your pocket all at once.

Property management: $240/month (10% of gross rent). Even if you plan to self-manage, price this in. Your time has a cost. And if you ever want to step away from day-to-day management, you need the numbers to work with a manager in place.

Now let's add it up:

$2,400 rent – $2,836 expenses = –$436/month

That duplex doesn't cash flow. It bleeds $436 every month. You'd be writing a check to own this property.

How Did $968 Turn Into Negative $436?

Because most investors skip five line items. They calculate rent minus mortgage and call it a day. The real expenses – vacancy, maintenance, CapEx, and management – eat up $864/month that simply didn't exist in the napkin math.

This is exactly how people end up as "accidental landlords" who hate real estate investing. They bought based on the top-line number, and the real costs slowly drained them.

What a Cash-Flowing Property Looks Like

For this duplex to break even, rents would need to be roughly $2,850/month total – about $1,425 per unit. To cash flow $200/month (a modest positive), you'd need rents around $3,050/month.

Alternatively, if you could buy the same property for $220,000 instead of $280,000, your mortgage drops to $1,022/month and total expenses fall to around $2,426. Now you'd cash flow about –$26/month. Still not great, but much closer.

The point isn't that you need to find magical deals. It's that you need to run the real numbers and know exactly where the break-even line is for any property you're evaluating.

The Metrics to Watch

Once you've calculated true cash flow, look at two additional numbers:

Cap rate. Take your annual net operating income (rent minus operating expenses, not including the mortgage) and divide by the purchase price. For this duplex: ($28,800 – $14,448) / $280,000 = 5.1%. That tells you the property's return regardless of financing. Useful for comparing deals.

Cash-on-cash return. Take your annual cash flow and divide by the total cash you invested (down payment + closing costs). If cash flow is negative, this number is negative too – a clear red flag.

Both metrics give you different angles on the same question: is this property actually making you money?

Stop Guessing

The difference between a good rental and a money pit is usually $200–400/month in expenses that investors forget to include. Don't be that investor.

Run your numbers through the [rental property calculator](/calculators/rental-property) and see the real cash flow – every expense included, no surprises.