To calculate NOI, take the property’s gross potential income, subtract vacancy and credit losses to get effective gross income, then subtract operating expenses. The formula is Effective Gross Income − Operating Expenses = Net Operating Income. Financing costs, capital projects, income taxes, and depreciation stay out on purpose, so the number measures the building’s earning power rather than the owner’s tax bracket or loan terms.
Step 1: Start With Gross Potential Income
Gross potential income is the maximum rent the property could collect if every unit were leased at current market rates for a full year. Ten units at $1,500 a month gives you $180,000. That’s the ceiling, not the deposit.
Add non-rent income the property actually generates: parking fees, laundry machines, pet rent, storage charges, vending. In multifamily buildings, even $50 per unit per month in ancillary fees adds up quickly.
Step 2: Subtract Vacancy and Credit Losses
No property runs at 100% occupancy with every tenant current. Vacancy loss covers units sitting empty between tenants. Credit loss covers rent owed but never collected. Together they turn a theoretical ceiling into effective gross income, which is what you can realistically expect to bank.
Most underwriters use a vacancy and credit loss factor between 5% and 10% of gross potential income. The right figure depends on the property’s history and the submarket. A building that has held 97% occupancy for three years in a tight market can justify 3%; a property with heavy turnover or rising area vacancies should sit closer to 8% or higher. Using zero is the fastest way to lose a lender’s confidence.
Step 3: Add Up Operating Expenses
Operating expenses are the recurring costs of keeping the property running, safe, and legally compliant. They show up whether the building carries a mortgage or not, and they stay reasonably steady year to year. The usual categories:
- Property taxes, typically the single largest line
- Insurance premiums, including any specialty coverage like flood or earthquake
- Property management fees, usually 8% to 12% of collected rent for residential
- Maintenance and repairs: plumbing, appliances, landscaping, pest control
- Utilities for common areas: hallways, parking, shared HVAC, trash
- Legal and accounting fees tied to leases, evictions, and property tax filings
- Advertising and leasing costs to fill vacancies
IRS Publication 527 lists deductible rental expenses that closely mirror these categories, including management fees, insurance, taxes, maintenance, and utilities.1Internal Revenue Service. Publication 527 (2025), Residential Rental Property – Section: Rental Expenses If an expense is deductible on Schedule E, it almost certainly belongs in NOI. The overlap isn’t perfect. Mortgage interest is deductible for taxes but excluded from NOI, and depreciation appears on the tax return but never in an operating statement.
Step 4: Run the Math
Subtract total operating expenses from effective gross income. The result is your net operating income.2J.P. Morgan. Calculating Net Operating Income in Multifamily Real Estate
A Worked Example
Take a 12-unit apartment building where each unit rents for $1,400 a month.
- Gross potential rental income: 12 × $1,400 × 12 = $201,600
- Other income (laundry, parking): $4,800
- Gross potential income: $206,400
- Vacancy and credit loss at 7%: −$14,448
- Effective gross income: $191,952
Now the expense side:
- Property taxes: $24,000
- Insurance: $6,500
- Property management at 10%: $19,195
- Maintenance and repairs: $14,400
- Common-area utilities: $7,200
- Legal and accounting: $2,400
- Advertising: $1,200
- Total operating expenses: $74,895
NOI = $191,952 − $74,895 = $117,057.
That figure is what the building produces before any loan payments, capital projects, or income taxes. Two investors comparing the same property should arrive at roughly the same number regardless of how each one plans to finance it.
What NOI Leaves Out, and Why
Several real costs are deliberately excluded. The exclusions exist so the metric describes the property rather than the owner.
- Debt service. Two buyers can purchase the same building with entirely different loan terms and down payments. Including mortgage payments would make the property look stronger for a cash buyer and weaker for a leveraged one, even though nothing about the building changed.2J.P. Morgan. Calculating Net Operating Income in Multifamily Real Estate
- Capital expenditures. A roof replacement or full plumbing overhaul is a one-time project. Including a $90,000 roof in one year’s NOI would wreck that year and flatter every other one.
- Income taxes. An individual and a corporation face different rates on the same income. Excluding taxes keeps NOI comparable across ownership structures.
- Depreciation. A non-cash accounting deduction that reduces taxable income but doesn’t move cash. It has no place in an operating performance number.
Where Replacement Reserves Fit
Replacement reserves are annual set-asides for inevitable capital needs like roofs, HVAC systems, and parking lot resurfacing. They sit in an awkward space: not a current operating expense, not a one-time capital project. Where they land depends on who’s doing the math.
In a standard operating statement, reserves fall below the NOI line. Fannie Mae, Freddie Mac, and most CMBS lenders instead calculate “NOI after reserves” and size loans against that lower figure. When a seller’s NOI doesn’t match a lender’s underwritten NOI, reserves are often the reason. Always ask whether a quoted NOI is before or after reserves.
Pro Forma NOI vs. Actual NOI
Sellers and brokers often lead with pro forma NOI, projecting what the property could earn under optimistic assumptions about rent growth, vacancy, and expense control. Actual NOI, sometimes called trailing NOI, reflects what the property has demonstrably earned, usually over the most recent twelve months.
The gap between the two is where inexperienced buyers get burned. A pro forma might assume 8% rent increases after renovations and vacancy dropping to 3%, producing a number that justifies a high asking price. The trailing financials may tell a different story. Solid underwriting starts from the actual trailing twelve months as a baseline, then builds a separate pro forma using your own assumptions about what you can achieve after acquisition. A seller who won’t share trailing financials and offers only pro forma projections is telling you something.
How Lenders and Investors Use the Number
Cap Rate
The capitalization rate divides NOI by the property’s market value.3PNC Insights. Capitalization Rate: What It Is and How Its Calculated The 12-unit example above, valued at $1,500,000, produces a cap rate of about 7.8%. Higher cap rates generally signal higher expected return and higher risk; lower cap rates often reflect more stable properties in stronger markets.
Debt Service Coverage Ratio
Lenders divide NOI by annual debt service to calculate DSCR.4J.P. Morgan. How to Use the Debt Service Coverage Ratio in Real Estate A DSCR of 1.0 means the property’s income exactly covers its loan payments. With $117,057 in NOI against $90,000 in annual mortgage payments, DSCR is 1.30, meaning the property earns 30% more than it needs to service the debt. Lenders use that cushion to judge whether the building can absorb a rough year.
Documents That Back the Number Up
Lenders don’t take your word for NOI, and neither should you when evaluating a purchase. Two documents do the verification work. The rent roll lists every unit with current rent, market rent, lease dates, tenant names, and vacancy status. The trailing twelve-month operating statement, called a T-12, shows actual monthly income and expenses for the past year with NOI at the bottom. Fannie Mae and Freddie Mac both require these, and the T-12 should be refreshed monthly.
Honest Numbers Only
Inflating NOI on a loan application is a federal crime. Under 18 U.S.C. § 1014, knowingly making false statements or overvaluing property to influence a lending decision carries penalties of up to $1,000,000 in fines, up to 30 years in prison, or both.5Office of the Law Revision Counsel. 18 US Code 1014 – Loan and Credit Applications Generally Even short of criminal exposure, a lender that finds material misrepresentation can call the loan, refuse future financing, or pursue civil damages. Build NOI from verified rent rolls and operating statements, not from what you wish the property earned.