How to Calculate Rental Cash Flow (Monthly)

When you walk a property, the first thing you picture is the rent check that lands in your account each month. Yet that check is only the top of a cascade that ends with the cash‑flow number you use to decide whether the deal is worth the risk. This guide walks you through every link in the chain, using a single, fully worked example so you can see the arithmetic in action and then apply the same logic to any property you evaluate.

All figures below come from a $250,000 single‑family home that rents for $1,800 per month, financed with a 20 % down payment and a 30‑year loan at 7 % interest. The example is deliberately modest: it shows a negative cash flow, a DSCR below 1.0, and a break‑even rent that is higher than the current market rent. Those outcomes are common in many markets, especially when investors rely on “good vibes” instead of a disciplined spreadsheet.

The formula chain (rent → EGI → NOI → cash flow)

The cash‑flow chain is a series of simple equations that turn a rent figure into the bottom‑line profit (or loss) after debt service. The chain looks like this:

Each step uses only the numbers from the previous step, so you can verify your work at any point. Below we break down each component with the example numbers.

Step 1 — gross rent and the vacancy haircut

Start with the contract rent. For our property the lease is $1,800 per month.

Yellow = inputs · Green = calculated

Vacancy is the portion of the year you expect the unit to sit empty. A common rule‑of‑thumb for a stable market is 5 %.

Yellow = inputs · Green = calculated

Dividing the EGI by 12 gives the monthly effective rent of $1,710.

Step 2 — operating expenses, line by line

Operating expenses are the costs you must pay to keep the property functional and attractive to tenants. In our example we use the following line items, each expressed as an annual dollar amount:

All five items together consume $8,088 per year, or $674 per month.

Step 3 — NOI

Net Operating Income (NOI) is the profit before any financing costs. Subtract the total operating expenses from the EGI:

On a monthly basis that is $1,036.

The cap rate is derived directly from the price: $12,432 ÷ $250,000 = 0.0497, or roughly 5 %.

Step 4 — debt service

Debt service is the monthly principal‑and‑interest (P&I) payment on the loan. With a $200,000 loan at 7 % interest over 30 years, the payment formula is:

where P = $200,000, r = 7 % ÷ 12 = 0.0058333, n = 360 months. The calculation yields $1,330.60 per month, or $15,967 per year.

DSCR (Debt Service Coverage Ratio) is NOI ÷ Debt service = $12,432 ÷ $15,967 ≈ 0.78. A DSCR under 1.0 means the property does not generate enough operating income to cover the loan payment.

Step 5 — cash flow, monthly and annual

Finally, cash flow is simply NOI minus debt service:

The negative cash flow tells you that, after paying the mortgage, you would need to subsidize the property by about $295 each month.

Break‑even rent: what would save this deal

Break‑even rent is the rent level that would lift cash flow to zero (or, more precisely, lift NOI to equal debt service). Set NOI equal to the annual debt service and solve for the required EGI:

But the rent-scaled lines grow with the rent: vacancy 5% + maintenance 5% + management 8% + CapEx reserve 5% take 23% of gross rent, so each gross-rent dollar keeps $0.77 after them. Solving 0.77 x gross rent - $4,200 = $15,967:

That translates to about $2,180 per month. Compared with the actual rent of $1,800, you would need an extra $380 of rent to break even. Notice the gap ($380) is larger than the monthly cash-flow deficit ($294.60): each extra dollar of rent loses 23 cents to vacancy and rent-scaled expenses before it can cover the mortgage.

Read more about the mechanics of break‑even rent in our Break‑Even Rent guide.

What cash flow leaves out (and why that matters)

Cash flow is a powerful snapshot, but it does not capture every financial impact of ownership. The metric excludes:

• Tax benefits such as depreciation, which can turn a negative cash flow into a positive after‑tax cash flow.
• Appreciation or market‑driven equity gains, which affect long‑term returns but not month‑to‑month cash.
• Capital improvements that are not part of the routine CapEx reserve.
• Opportunity cost of the cash you have tied up in the down payment and closing costs.

Because cash flow ignores these items, investors often pair it with other metrics—cap rate, cash‑on‑cash, and internal rate of return (IRR)—to get a fuller picture. Our NOI guide and DSCR guide explain how those numbers complement cash flow.

Cash flow vs profit: why they differ

Cash flow is the money that actually lands in your pocket each month after you pay operating expenses and debt service. Profit, or total return, adds everything you gain over the holding period—including principal paydown and any increase in property value. Because the two concepts pull from different buckets, a property can bleed cash every month yet still post a positive return on paper.

In our $250,000, $1,800‑rent example the net operating income is $12,432 per year, but the debt service of $15,967 creates a negative cash flow of −$3,535 per year (about −$294.60 per month). The loan amortization contributes a principal paydown of $2,032 in the first year. If the market appreciates at 3 %, the property adds roughly $7,500 in equity. Adding those three pieces together gives a total return of −$3,535 + $2,032 + $7,500 = $5,997, which is a 10.9 % return on the $55,200 cash invested. Strip out appreciation and the math flips: −$1,503 total, or −2.7 %.

Even though the headline number looks attractive, the negative cash flow means you must cover the shortfall out of pocket each month. Relying on appreciation to “rescue” the deal is risky because market growth is not guaranteed. A solid investment should generate positive cash flow on its own; otherwise the upside from equity buildup is merely a speculative bonus, not a substitute for sustainable income.

Run your own numbers

The workflow above is exactly what our spreadsheet automates. Start with the free Lite version of the Real Estate Deal Analyzer — the main analyzer sheet with cap rate, NOI, cash flow, cash‑on‑cash, DSCR and the 1 % rule check. If you screen deals regularly, the Full version adds a sensitivity grid, a 5‑year projection with IRR, a BRRRR calculator, a 3‑deal comparison sheet, and a 15‑point due‑diligence checklist. Both are on the Real Estate Deal Analyzer page.

This guide is an educational calculation aid, not financial, tax, or legal advice. Verify all figures against local data and consult licensed professionals before purchasing any property.