NOI (Net Operating Income)

Net operating income — NOI — is what’s left of a building’s rental income after the costs of running it are subtracted, and before anything related to how it was financed or owned is factored in. Take gross rental income, subtract operating expenses like property management, maintenance, insurance, and property taxes, and what remains is NOI. Notice what’s deliberately missing from that subtraction: mortgage interest, depreciation, and capital expenditures. Those are excluded on purpose, because NOI is meant to answer a narrower, cleaner question — not “how much money does the owner keep,” but “how much does this asset itself earn, independent of who owns it or how they paid for it.”

That distinction matters more than it sounds like it should. Two identical buildings, side by side, generating the same rents and facing the same expenses, will have the same NOI even if one owner financed the purchase with 80% debt and the other paid all cash. Their personal cash flows will look completely different — the leveraged owner’s income eaten up by mortgage payments, the cash buyer’s income intact — but the buildings themselves are earning exactly the same amount. NOI strips financing out of the picture so that buildings, not balance sheets, can be compared apples to apples.

This is also why NOI sits at the center of nearly every other calculation in commercial real estate. Cap rate is NOI divided by price. Debt service coverage ratio compares NOI to the annual mortgage payment a lender expects to receive. Loan sizing, in many commercial deals, starts not with the buyer’s credit score but with NOI — lenders want to know the property can service the debt out of its own earnings, regardless of who’s signing the papers. Change the NOI, even slightly, and every number downstream of it moves. This is why serious buyers spend so much of their diligence time picking apart a seller’s NOI claims line by line rather than accepting the headline figure.

That scrutiny exists for a reason: NOI is also the easiest number in the deal to quietly inflate. A seller can pad NOI by underspending on maintenance the year before a sale, deferring repairs that will become the buyer’s problem later. They can classify one-time income — a lease termination fee, a temporary rent bump — as if it were recurring. They can under-budget for a vacancy allowance that any sober operator knows is coming. None of these tricks are illegal; they’re simply optimistic. A buyer’s job is to rebuild the NOI figure from the ground up, using realistic, sustainable assumptions, rather than trusting the seller’s version of the story.

Operators sometimes distinguish between “in-place NOI” — what the building is earning right now, under its current leases and current management — and “stabilized NOI” — what it’s projected to earn once renovations are finished, vacant space is leased up, and rents are pushed to market. The gap between those two numbers is often the entire investment thesis of a value-add deal: buy at a price based on today’s modest NOI, do the work, and sell — or refinance — based on tomorrow’s higher one.

If cap rate is a building’s price tag, NOI is its salary. Get the salary wrong, and every price tag built on top of it is wrong too.