Net operating income (NOI) is what a property earns from operations. Take the rental income you actually collect over a year and subtract ordinary operating expenses — management, insurance, property taxes, maintenance, utilities you pay, and a realistic vacancy allowance. What remains is NOI.
Critically, NOI excludes your mortgage (debt service), depreciation, and income taxes, so it describes the property’s own earning power rather than your financing. It’s the building block for cap rate and several other metrics, which means it’s only as trustworthy as the per-property books behind it.