Rental property cash flow formula: from NOI to cash-on-cash

The formula in one line

Annual cash flow = net operating income (NOI) − annual debt service

That is the core. Everything else is about building NOI correctly, calculating the loan payment, and deciding which cash outlays belong in the denominator of the return.

The monthly figure is usually what investors quote:

Monthly cash flow = annual cash flow ÷ 12

For a leveraged property, cash-on-cash return is:

Cash-on-cash return = annual pre-tax cash flow ÷ cash invested × 100

Each part has a specific job. Mixing them up is the most common source of a cash flow number that looks better than the deal.

Step 1: Build gross scheduled income

Gross scheduled income (GSI) is the rent the property would collect over a year if it stayed occupied, plus recurring other income such as parking, laundry, pet rent, or a utility reimbursement.

GSI = 12 × (monthly rent + other monthly income)

Use the market rent you can support with evidence, not the rent the seller hopes to get. If the property is vacant, a rent comp is more useful than a pro forma. If it is occupied below market, decide whether you are underwriting the current rent or a market rent after turnover. Do not use the higher number without a plan and a cost for getting there.

Step 2: Subtract vacancy and credit loss

Vacancy is not only an empty unit. It also includes the gap between leases, turnover time, and rent that is never collected.

Effective gross income (EGI) = GSI − (GSI × vacancy and credit loss rate)

A common screening assumption is 5% for a stable property, but it is not a law. A single-family rental with one tenant has binary occupancy: the property is either 100% occupied or 0%. A 5% annual vacancy assumption on a single unit is really a reserve for turnover, not a guarantee that the unit will be empty for 18 days every year.

For a small multifamily property, look at actual trailing vacancy and the local rent roll. For a first purchase, stress the vacancy rate above your base case.

Step 3: Subtract operating expenses

Operating expenses are the ordinary costs of keeping the property rented and running. They generally include:

• Property taxes
• Landlord insurance
• HOA dues, if any
• Property management
• Maintenance and routine repairs
• Leasing costs and advertising
• Utilities paid by the owner
• Lawn care, pest control, and other recurring services
• A capital expenditure reserve, if you choose to include one
• A separate reserve for major systems, if you choose to include one

Management, maintenance, and capex are often entered as percentages of gross scheduled income. Taxes, insurance, HOA dues, and utilities are usually entered as fixed dollar amounts.

Do not include mortgage principal and interest, income taxes, depreciation, or a major renovation in operating expenses. Those belong somewhere else in the analysis, or outside it.

NOI = EGI − operating expenses

NOI is a property-level number. It does not care how the property is financed. That is what makes it useful for comparing properties and for calculating cap rate.

Step 4: Calculate annual debt service

Debt service is the cash required to service the loan. For an amortizing mortgage, the monthly payment includes principal and interest. If the loan requires mortgage insurance, add that monthly amount.

Annual debt service = 12 × (monthly principal and interest + monthly PMI)

The monthly principal and interest payment comes from the loan amount, interest rate, and amortization term. A longer term lowers the payment but increases the total interest paid. A shorter term does the opposite.

Do not add property taxes and insurance to debt service if you already included them in operating expenses. In an escrow account, the lender collects those costs as part of the total housing payment, but they are still operating expenses for the property analysis. Count them once.

Step 5: Calculate cash invested

Cash invested is the money that comes out of your pocket to acquire and stabilize the property. It usually includes:

• Down payment
• Closing costs
• Immediate rehab or make-ready costs
• Lender fees and points
• Any reserves required by the lender

It does not include the full purchase price when you are using a loan, because the lender provides the rest. It also does not include future capital expenditures that have not happened yet, unless you choose to reserve for them up front.

Cash invested = down payment + closing costs + immediate rehab + lender fees + required reserves

Be consistent. If you include a repair in operating expenses, do not also add it to cash invested. If it is a capital improvement that you pay for at closing, put it in cash invested and keep it out of NOI.

Step 6: Solve for cash flow and cash-on-cash return

Now the two numbers fit together:

Annual cash flow = NOI − annual debt service

Monthly cash flow = annual cash flow ÷ 12

Cash-on-cash return = annual cash flow ÷ cash invested × 100

Cash flow tells you whether the property pays its bills and puts money in your pocket. Cash-on-cash tells you how hard the cash you invested is working.

A property can have positive cash flow and a modest cash-on-cash return if you put a lot of cash down. Another property can have a higher cash-on-cash return with less cash down but more risk. The metric is a starting point, not a verdict.

Step 7: Check DSCR

The debt service coverage ratio (DSCR) measures how much NOI exceeds the debt service:

DSCR = NOI ÷ annual debt service

A DSCR of 1.00 means NOI exactly covers debt service. A DSCR below 1.00 means the property does not cover its mortgage from operations, even before income taxes and capital events. A DSCR above 1.00 provides a cushion, but the size of a comfortable cushion depends on the property, the loan, and the lender.

DSCR is especially useful because it is independent of the down payment. Two buyers can have different cash-on-cash returns on the same property because they financed it differently, but the property’s NOI and debt service produce one DSCR for those loan terms.

Worked example

Consider a $300,000 single-family rental with $2,600 monthly rent and no other income:

Line item	Amount
Gross scheduled income	$31,200
Vacancy and credit loss at 5%	−$1,560
Effective gross income	$29,640
Property taxes	−$3,600
Landlord insurance	−$1,800
Management at 8% of GSI	−$2,496
Maintenance at 5% of GSI	−$1,560
Capex reserve at 5% of GSI	−$1,560
Net operating income	$18,624

Now add a 25% down payment, a $225,000 loan at 7% for 30 years, and no PMI:

Financing line	Amount
Monthly principal and interest	$1,496.93
Annual debt service	$17,963.17
Annual cash flow	$660.83
Monthly cash flow	$55.07
Cash invested (25% down)	$75,000
Cash-on-cash return	0.88%
DSCR	1.04

This is a thin deal. The property covers its debt service, but a small change in vacancy, taxes, insurance, or the interest rate can erase the cash flow. That is exactly the kind of result a stress test is for.

The cap rate on the same operating assumptions is 6.21%. The cap rate is unchanged by the loan. The cash-on-cash return is not.

Common mistakes

Using gross rent as cash flow

Gross rent ignores vacancy, taxes, insurance, management, maintenance, and capex. It is a revenue line, not a return.

Putting the mortgage inside operating expenses

Debt service is not an operating expense. If you subtract it while calculating NOI and then subtract it again to get cash flow, you double count it. Keep NOI unleveraged and put debt service below it.

Treating capex as optional

Roofs, HVAC systems, water heaters, and siding are real costs. A reserve is an estimate, not a guarantee, but pretending the cost is zero makes the cash flow look better than the property is.

Counting principal paydown as cash flow

Principal paydown increases equity. It is not cash available to pay bills or distribute to investors. Show it separately if you are analyzing total return.

Forgetting PMI or lender reserves

Mortgage insurance and required reserves affect cash invested and monthly cash flow. Leaving them out overstates the return.

Using a single optimistic scenario

A base case is not a stress test. Raise vacancy, lower rent, increase taxes and insurance, and test a higher interest rate. The goal is not to find the worst possible outcome. It is to see whether the deal still works when one assumption is wrong.

Comparing cash-on-cash returns with different assumptions

A 12% cash-on-cash return with 3% vacancy and no management is not comparable to an 8% return with 8% vacancy and professional management. Put the deals on the same underwriting basis before ranking them.

How to use the calculator

The rental cash flow calculator follows the sequence above. Enter the property income and expenses, choose the financing assumptions, and review the monthly and annual cash flow, cash-on-cash return, DSCR, break-even rent, and five-year projection.

Use the sensitivity table to see how cash flow changes when rent and vacancy move together. Use the reverse solves to answer a narrower question: what rent or purchase price makes the deal hit the target? Then compare the answer with the market evidence.

If you want the unleveraged view first, run the cap rate calculator. It shows the property-level yield before financing. The methodology page documents how the two models connect.

The bottom line

Rental cash flow is not a single number pulled from a listing. It is a sequence: income, vacancy, operating expenses, NOI, debt service, cash flow, and cash-on-cash return. Each step has a job. When the sequence is clear, the result is easier to challenge, which is the point.

A good cash flow result is durable, not just positive. Check the assumptions, stress the deal, and decide whether the cash flow compensates you for the risk you are taking.