NOI.
What it actually means.

What it is

Net operating income is what a property earns as a business, before any decision about how to pay for it. Excluding the mortgage is the entire point: two investors can buy the same building with completely different loans, and NOI stays identical because it describes the asset rather than the balance sheet. Income taxes are excluded for the same reason, since they depend on the owner rather than the property. What remains is the cleanest available measure of operating performance, and nearly every valuation and lending metric in income real estate is built on top of it.

How it is calculated

Start with gross scheduled rent, subtract vacancy and any collection loss to get effective gross income, then subtract operating expenses: property taxes, insurance, management, repairs and maintenance, utilities the owner pays, and a capital reserve. Do not subtract principal, interest, depreciation, or income taxes. A property grossing $18,000 with $6,000 of operating expenses produces $12,000 of NOI. Underwriting NOI honestly means using the reassessed tax bill rather than the seller’s, a real insurance quote, and a management line even when you intend to self-manage — your time is a cost whether or not it is invoiced.

NOI = effective gross income − operating expenses. Debt service and income taxes are excluded. Rent of $18,000 less $6,000 of operating expenses gives $12,000.

How investors actually use it

NOI is the input to cap rate, and through cap rate it drives value directly. In a market pricing at a 7% cap rate, each additional dollar of durable NOI adds roughly fourteen dollars of value, which is why operators pursue permanent income gains and permanent expense reductions rather than one-time savings. The effect is real but easy to overstate: raising rent $50 across four units is $2,400 of gross annual rent, and after vacancy and management on that incremental rent it is closer to $2,000 of NOI — about $29,000 of value at a 7% cap, not the full gross capitalized.

The common mistake

Slipping the mortgage into the expense list. Including debt service produces a number that is neither NOI nor cash flow, and running it through a cap rate calculation values the property at a fraction of what it is worth. The mirror-image error is omitting the unglamorous operating costs — vacancy, management, and capital reserves — which inflates NOI and, because value is NOI divided by a small decimal, inflates apparent value by roughly fourteen times the omission.