Net Operating Income is the profit a rental property generates from operations alone, before you account for mortgage payments, capital improvements, or income taxes. It is the single most important number in commercial real estate valuation because it isolates the performance of the asset itself from how it is financed or owned.
To calculate NOI, start with gross scheduled rent, subtract vacancy and collection loss to get effective gross income, then subtract all operating expenses: property management fees, repairs and maintenance, property taxes, insurance, utilities you pay, landscaping, HOA dues, and turnover costs. Do not subtract mortgage interest, principal, depreciation, or capital expenditures. Those live below the NOI line.
NOI drives cap rate, debt service coverage, and most bank underwriting. When you grow NOI on a property you already own, you grow both the cash flow and the appraised value at the same time.
Example
A duplex collecting $36,000 per year with $4,200 in vacancy/collection loss, $8,400 in operating expenses produces NOI of $23,400.