How to Calculate Rental Cash Flow — Expenses Beginners Miss

How to actually calculate rental cash flow

If you want a real answer to “Will this property cash flow?” you must stop at the single rent-minus-mortgage number. That number ignores the recurring and intermittent expenses that consume profits. Below is a step-by-step method and a worked example that exposes the lines beginners forget: capex, vacancy, professional management, turnover, and other soft costs.

Step-by-step: build the true cash-flow calculation

1. Start with Gross Scheduled Rent (GSR): the total contract rent if all units are occupied.
2. Subtract your vacancy allowance to get Effective Gross Income (EGI). Vacancy is not zero — use a realistic percentage (5–10% depending on market).
3. Subtract operating expenses (not the mortgage). Include every line below.
4. The result is Net Operating Income (NOI). Subtract the mortgage P&I to get monthly cash flow.

Expense lines beginners skip

• Property tax (annual, divided by 12). Don't estimate a round number — get the assessed/estimated tax bill.
• Insurance (landlord policy, not homeowner).
• Property management fees (typical 6–12% of rent for local managers; 8–10% is common).
• Maintenance and routine repairs (budget 5–10% of rent or more for older homes).
• Capex reserve (roof, HVAC, appliances). Use 5–10% of rent or a fixed amount like $150–300/month depending on property age.
• Vacancy reserve (5–10% of rent).
• Turnover and leasing costs (advertising, painting, cleaning, and potential leasing commissions). Estimate turnover cost and amortize it monthly (e.g., $2,400 per turnover every 24 months → $100/month).
• HOA fees, utilities (if owner-paid), garbage, landscaping, pest control.
• Accounting, legal, software subscriptions, licensing.
• Unexpected items and contingency ($25–100/month depending on scale).

Treat capex and turnover as real monthly costs by putting them into a reserve. They are not optional.

Worked example: duplex listed at $320,000

Say a duplex lists at $320,000 and you can rent both sides for $1,200 each: Gross Scheduled Rent = $2,400/month.

Assumptions you must write into your model:

• Vacancy: 8% → $2,400 × 0.08 = $192/month
• Effective Gross Income: $2,400 − $192 = $2,208/month
• Loan: 20% down → $64,000 down, loan = $256,000 at 4.5% for 30 years → P&I ≈ $1,298/month

Now the operating expense breakdown (monthly):

• Property tax (estimate 1.2% annually of value = $3,840/yr) → $320
• Insurance → $100
• Maintenance & repairs (10% of rent) → $240
• Capex reserve (5% of rent) → $120
• Property management (8% of rent) → $192
• Turnover/leasing reserve (one turnover every 24 months at $500) → $21
• Accounting/other → $25

Total operating expenses (excluding mortgage) = $1,017/month

NOI = EGI − operating expenses = $2,208 − $1,017 = $1,191/month

Cash flow = NOI − mortgage P&I = $1,191 − $1,298 = −$107/month

If you had omitted capex and management (two very common blind spots), your non-mortgage expenses would drop by $312/month. That would show NOI = $1,503 and cash flow = $205/month — a seemingly positive deal. But the real outcome will be closer to the negative number once those costs arrive.

What to do if the cash flow is negative

• Increase down payment to lower the mortgage. In the example, using 30% down (loan $224,000) reduces P&I to ≈ $1,136 and yields about $55/month positive cash flow.
• Negotiate price down, which reduces taxes and mortgage.
• Increase rents only if market supports it — do not assume instant rent bumps.
• Cut management fees by self-managing initially (but factor your time cost).
• Target properties with lower maintenance or more recent capex.

Quick checklist before you buy

• Calculate cash flow using EGI (after vacancy), not gross rent.
• Always include a capex reserve and a turnover/leasing reserve.
• Include management fees if you’re not doing the work.
• Use actual projected tax and insurance numbers rather than round estimates.
• Stress-test with higher vacancy and higher repair reserves.

If you want to run these inputs quickly and compare scenarios (different down payments, vacancy, and capex), try the rental cash flow calculator to see how each line changes the outcome.

Final note

A “positive on paper” number that ignores capex, vacancy, and management is a mirage. Build reserves, be conservative on rent and vacancy, and always run a mortgage and expense stress-test before you sign. Now run the numbers on the next listing and see what the real cash flow looks like.