How to Calculate Rental Property Cash Flow (Monthly)

How to Calculate Rental Property Cash Flow

To calculate rental property cash flow, subtract every operating expense and your debt service from your effective monthly rental income. The formula is simple: Cash Flow = Gross Rent − Vacancy Loss − Operating Expenses − Debt Service. What separates a real number from a fantasy is whether you actually account for all of those expenses — not just the mortgage and the rent.

Most bad deals don't look bad on paper because the math is hard. They look good because the investor left expenses out. This guide walks you through the full calculation line by line, using a concrete $220,000 single-family rental as the running example.

Start With Effective Gross Income, Not Asking Rent

Your top line is not the rent number in the listing. It's the rent you'll realistically collect after the unit sits empty between tenants and after occasional non-payment.

• Gross scheduled rent: $1,800/month → $21,600/year
• Vacancy allowance (7%): −$1,512/year
• Effective gross income: ~$20,088/year, or $1,674/month

A 5–8% vacancy factor is a reasonable starting point in a stable market, but it swings with location and property class. A B-class suburban rental might run 5%; a rougher C-class area with higher tenant turnover can run 10% or more. Pull local vacancy data rather than guessing — this single number moves your cash flow more than most people expect.

The Operating Expenses People Forget

Here's where projections quietly fall apart. The mortgage is the obvious cost. The operating expenses are the ones that get "forgotten" until they show up as a surprise. On the $1,674/month effective income, budget for:

• Property taxes — $250/month ($3,000/year). Verify the actual assessed amount, and remember taxes often reset after a sale.
• Insurance — $110/month ($1,320/year).
• Property management — $167/month (10% of collected rent). Include this even if you self-manage; your time has a cost, and you may hire out later.
• Repairs & maintenance — $134/month (~8% of rent). Older homes trend higher.
• Capital expenditures (CapEx) — $134/month (~8%). This is your reserve for the roof, HVAC, and water heater — big-ticket items that don't hit every month but always eventually hit.
• Vacancy — already accounted for above.

That's roughly $795/month in operating expenses before you ever touch the loan. This is exactly the intuition behind the 50% rule for rentals — that operating costs plus reserves tend to eat around half of gross rent over time.

CapEx is not optional

The single most common cash flow mistake is skipping CapEx reserves. A roof might cost $9,000 and last 20 years, but it doesn't wait until you're ready. If you skip the reserve, your spreadsheet shows $200/month of "cash flow" that a single furnace replacement erases for two years. Budget it monthly whether or not you spend it.

Add Debt Service, Then Do the Math

Now layer in the mortgage. Assume 20% down on the $220,000 purchase ($44,000), financing $176,000 at 7.25% over 30 years.

• Principal & interest: ~$1,201/month

Put it all together:

That's the honest result at these numbers: negative cash flow. The middle line — net operating income — is positive at $879, which is why deals that fail on cash flow can still look healthy if you stop at NOI. The mortgage is what tips this one underwater.

This is not a reason to panic; it's the point of running the numbers. Now you know your levers.

The Levers That Turn a Negative Into a Positive

When a deal comes back negative, you have four honest ways to fix it — not by deleting expenses, but by changing the deal:

• Buy it cheaper. Dropping the price to $200,000 cuts the loan and P&I. A max-offer that produces your target cash flow is a number you can calculate and then negotiate toward.
• Put more down. Going to 25% down lowers the payment but also lowers your cash-on-cash return — more cash in for less relative yield. Weigh both.
• Raise the rent. If comparable units command $1,950, the math changes fast. But verify with real rent comps, not optimism.
• Reduce financing cost. A lower rate, seller financing, or an assumable loan can swing the payment meaningfully.

The worst "fix" is pretending expenses don't exist. Zeroing out CapEx and management to force a positive number doesn't make the deal profitable — it just delays the day you find out it wasn't.

Why Manual Cash Flow Math Goes Wrong

Every number above depends on inputs: local vacancy, actual taxes, market rent, realistic insurance, current rates. Guess any of them and your cash flow figure is fiction — garbage in, garbage out. The tedious part isn't the arithmetic; it's sourcing accurate, current data for each line.

This is where analyzing deals against real data changes the game. Instead of typing in assumptions, tools like PropertyWiz AI pull live, verified market value, rent, taxes, insurance, and vacancy the moment you load a property, then stress-test the cash flow automatically — so you're deciding, not guessing. If you want a broader step-by-step, see how to analyze a rental property in under 30 minutes.

Cash Flow Best Practices

• Underwrite conservatively. If it only works with best-case rent and no vacancy, it doesn't work.
• Always reserve for CapEx and maintenance, even on new construction.
• Verify taxes and insurance for the specific property — regional averages hide big swings.
• Track cash-on-cash, not just monthly dollars. $200/month on $40,000 invested is a 6% return; on $10,000 it's 24%.
• Re-run the numbers before you offer. Rates and rents move; a projection from three months ago may be stale.

Get these lines right and cash flow stops being a hopeful estimate and becomes a decision you can defend.